Since the question is about how to improve the marshal, makes the most sense to start with that, and other classes that can complement and synergize with the marshal.
Marshal (Miniatures Handbook)
The marshal has very, very limited opportunity to optimize. They only have four class features:
Skill Focus (Diplomacy)
This is just a feat, and a fairly basic one at that. There are loads of things you can do to improve Diplomacy, and taking Skill Focus would be a step most diplomancers will likely take, so hey, you get it free. But it’s not really “special.” You optimize it by optimizing Diplomacy.
Note that Diplomacy is a really problematic skill. As written, high Diplomacy checks can allow you to just tell people what to do, even if they should hate you. Hence the tongue-in-cheek “diplomancer” to refer to characters who have very high bonuses. No one runs it like that, or should—but as a result, it becomes very unclear what a high Diplomacy check should get you.
At any rate, optimizing a diplomancer has little to do with the marshal class—the only relation is that marshal could be a source of a feat that a diplomancer probably wants. Look up any of the many Diplomacy/diplomancy handbooks out there for more on how to get the most out of the skill; it isn’t hard to break the game, as written.
Super-niche, but there is another application of this class feature that actually is unique to the marshal, not that I’ve ever found a particularly good use for it—if you already have Skill Focus (Diplomacy) when you take marshal 1st, you get to choose another feat you qualify for. This is actually very rare, officially, though many if not most DMs just allow it for all bonus feats. Anyway, you can leverage this if for some reason you want to “delay” a feat you otherwise could have chosen until later, presumably because the feat you want requires things you don’t have yet. You can take Skill Focus (Diplomacy) instead, and then take your 1st level of marshal when you want to get the other feat.
This is probably the best class feature that the marshal has. It’s certainly the one most amenable to optimization, which is to say it can be optimized, at all, because none of the others really can.
The idea with your minor aura is to pump your Charisma as high as possible, so that when you, say, motivate Dexterity, you’re offering a large bonus.
Dexterity is, by the way, probably the best target: since initiative is a Dexterity check, motivate Dexterity applies to it, and giving +Cha to your entire team’s initiative checks is amazing. Initiative is often the most important roll in a battle, so that’s nothing to sneeze at.
Master of tactics and the various save boosters aren’t bad, either—sadly, you can’t ever have more than one minor aura active at a time.
Since optimizing this means pumping Charisma as hard as you can, it is very, very worthwhile to look for other opportunities to accomplish “Cha SAD-ness,” where “SAD” means “single attribute dependency.” In other words, since you are getting a stratospheric Charisma score, you want to apply that score to as many things as you possibly can. Unfortunately, minor aura is the only marshal feature that accomplishes that, and can only do it for one thing at a time.
Ironically, this isn’t as good as the minor aura. Charisma can be huge; this bonus starts at +1 and grows extremely slowly.
The save bonus is almost-certainly the best target for it. The DR isn’t bad—particularly at low levels—but ultimately the saves are more important.
You can’t have more than one major aura active at a time.
There are a few feats that improve auras, but they’re basically uniformly garbage. So the optimal thing to do here is basically not do anything, because putting resources into this feature is largely wasted.
The only non-epic aura-improving feat that is even kind of good is Dilate Aura (Fiendish Codex II: Tyrants of the Nine Hells)—that lets you double the range of “your aura” for Cha bonus rounds, once per encounter. With your Charisma, that should easily mean the entire encounter. On the other hand, marshal auras have a staggering 60-foot radius, which is also going to easily cover the vast majority of battlefields.
Also, note that it says “your aura”—no word on what happens if you have more than one, e.g. a minor and a major. The feat also says nothing about what kind of action it takes to activate this feat. If it takes more than a swift, this should be a hard-pass. If it’s swift, it’s iffy, but maybe depending on what else you’re doing with swift actions and how often you think battles might use it. If it’s free, then it’s looking pretty decent−if you expect some wide-open combat scenarios.
Dragon Magic page 86 states that a marshal can take a draconic aura in place of a major aura, and if they do, it follows the rules for major auras. This does increase the range (draconic auras default to a 30-foot radius), but it also means that you can’t have the draconic aura up at the same time as your own major aura—which pretty much ruins the option.
That isn’t to say that draconic auras are no good—some are decent. You’re just better off getting them through the Draconic Aura (Dragon Magic) feat, so they can remain independent of your marshal auras and you can use both at once. As long as you have the dragonblood subtype—which isn’t hard to get and you probably will want anyway—the feat version also scales about the same as your major auras, but based on character level instead of marshal class level. That can be a big win. Of course, at this point, being a marshal has nothing to do with your draconic auras since Draconic Aura just requires character level 3rd, so this isn’t really optimizing “marshal” at this point so much as it is “something anyone can do, but a marshal might be somewhat more interested in than others.”
Anyway, there are some relatively good options for draconic auras, especially in a more magical party. I imagine your allies may particularly like energy (Dragon Magic), power (Dragon Magic; note that Player’s Handbook II has an entirely different aura named power), and resolve (Dragon Magic), though senses (Player’s Handbook II or Dragon Magic) and vigor (Player’s Handbook II) are decent for any party really. Honestly, there are enough decent draconic auras that I’d consider recommending dragon shaman for picking them up, except that you’re still limited to one or at most two at a time (and if you get them via dragon shaman, they scale based on dragon shaman level and not character level).
Finally, a really key thing about draconic auras is that Double Draconic Aura (Dragon Magic) exists. That’s pretty substantial; I could easily see 2×Draconic Aura + Double Draconic Aura being worthwhile to you.
Grant move action
This is great!
But you only get it a very few times per day.
There isn’t really any way to “optimize” this, save for using it on the allies who will get the most use from it. That would usually be anyone who wants to full-attack, but doesn’t have anyone in reach.
So most of these can’t really be optimized in any meaningful way. The ones that can—kinda—are basically potential building blocks for optimizing other things (Diplomacy, Cha SADness), but aren’t things the marshal is particularly good at alone. More importantly, you get all of these abilities by 4th level—and you get the best, most optimizable one, at 1st. More levels let you learn more auras—but you can still only have one minor and one major aura at a time, which really limits the value of learning your second- or third-favorite aura of each type. Major aura also grows in size, but extremely slowly—taking 5–7 levels just to get a +1 bonus is not an optimal strategy by any means. More uses of grant move action would be nice, but at the rate you get them, it’s too much.
All told, it’s really hard to make marshal more than 1–4 level dip. Maybe 5th, just because you get a 3rd minor and a 2nd major aura, though again those are going to be also-rans against what you already have. With your party—which is heavily magical and not looking for full-attacks—even 4th level seems hard to justify. If one of those casters is playing a summoner, maybe?
And to put marshal to the best use, we need other class features—because we want to go for Charisma for everything.
Bard (Player’s Handbook)
An obvious choice, bards’ music works a lot like marshals’ auras, and fill a similar niche. Note that bardic music doesn’t actually require music per se, just an audible performance, so no worries about having to turn into a musician—Perform (oratory) should be just fine. Unlike auras, bardic music is very, very amenable to optimization—medallion of courage (Magic Item Compendium), inspirational boost (Spell Compendium), and/or Song of the Heart (Eberron Campaign Setting) can easily get your inspire courage up to +3 or +4 even with just a few (1, 2, or 3, respectively) levels of bard. Words of Creation (Book of Exalted Deeds) can double that, though honestly I’d suggest that may be going too far.
Dragonfire Inspiration (Dragon Magic) grants a new song that adds Xd6 fire damage to allies’ attacks, where X is the bonus from inspire courage—meaning all of the above optimization applies to that as well. Since it’s a separate song, you can play one, stop, and play the other, and the two overlap while the first lingers and your party gets +X attack and +Xd6+X damage, which is rather impressive. (Some DMs may decide that the two are too closely related to stack them like that, though; RAW, it’s good, but it is a bit of a technicality. If yours makes that ruling, Dragonfire Inspiration can still be good but you might be better off just saving the feat. If your allies have Power Attack inspire courage might even be the better song.)
Song of the White Raven (Tome of Battle) lets you start inspire courage as a swift action. This is amazing, though qualifying as a bard/marshal is hard—more on that in a bit.
Snowflake Wardance (Frostburn), though, is the real reason we are here. This feat gives us a new usage of bardic music that allows us to add Charisma to attack rolls with slashing weapons used in one hand. This is great for us, because it allows us to use our Charisma for one of the big important things we want to do.
Paladin (Player’s Handbook)
You want to focus on Charisma? Then if you don’t have divine grace, you have made a mistake somewhere. Divine grace is arguably the best non-scaling class feature in the game. It is preposterous how strong divine grace can make your defenses. A Charisma-heavy paladin can actually make an excellent off-trapmonkey by just walking through the traps and almost-certainly making all the saves.
Conveniently, there is a variant paladin, the harmonious knight (Champions of Valor web enhancement), that gets inspire courage +1, 1/day at 1st level, instead of at-will detect evil. Not saying that detect evil isn’t nice, but considering everything we just said above about inspire courage, I hope it’s clear that inspire courage is a whole lot better. Note that to be a harmonious knight, you need to worship Milil—at least, you do in the Forgotten Realms. As Champions of Valor is a Realms-specific book, in other settings your DM may adapt it to another god—or just ban it entirely. Also, technically, the harmonious knight gets “inspire courage” but not “bardic music”—their inspire courage ability doesn’t reference bardic music or the bard class in any way. That might be an issue qualifying for bardic music feats, which is largely the point of taking harmonious knight in the first place. So ask your DM about those things.
If your DM is on board with harmonious knight and with its inspire courage counting as bardic music for prerequisites, you don’t actually need to take bard levels at all. You’ll probably want to take Extra Music (Complete Adventurer), though—each one gives you 4 additional uses per day, so even taking it once should just about do you, unless you really want to layer inspire courage and Dragonfire Inspiration and Snowflake Wardance on every fight.
If your DM is not OK with harmonious knight, or rules that it doesn’t meet “bardic music” prerequisites, they might find From Smite to Song (Champions of Valor, the book this time) more convincing:
You can spend a smite evil attempt to duplicate the effects of the inspire courage bardic music ability (see page 29 of the Player’s Handbook). Your effective bard level (for the purpose of determining the value of the bonuses granted) is equal to your paladin level. Add Perform to your list of paladin class skills.
This explicitly references bardic music, and says you count as a bard of your paladin level for the purposes of the bonus granted by your inspire courage bardic music effect. It’s still a little iffy since you are “duplicating the effect of” rather than just having this ability, but if you value a level more than a feat (and you generally should), it can be worth asking.
Otherwise, you can just take a level of bard and a couple of levels of paladin. It’s not as though the bard level is bad, we just were trying to economize a little.
Anyway, regardless, paladin is a 2-level class. We get divine grace and get out.
any class that can get Charisma to AC (various)
As long as we’re getting Charisma to our saves, how about that other defense, AC? Monk, obviously, has Wisdom to AC while unarmored—you can make that Charisma using Ascetic Mage (Complete Adventurer). But monk isn’t the only class to get Wisdom to AC, and Ascetic Mage works on any of them. You could also do “swift and deadly hunter” druid, moon-warded ranger (Dragon vol. 340), or swordsage (Tome of Battle) are all options as well—and the ranger or swordsage get to use light armor and still get the bonus (!), though they each take two levels to get it.
If you don’t want to spend the feat, or can’t qualify (e.g. didn’t take bard levels), battledancer (Dragon Compendium) has Charisma to AC while unarmored without taking any feat. Battledancer is a lot worse than monk (and that’s saying something), but depending on your build getting it cheap might be better than jumping through hoops.
Crusader (Tome of Battle)
And here we are, my real answer to this question: everything the marshal was trying to do, the crusader does better. The White Raven discipline is all about battlefield leadership, and it’s excellent. Where the marshal is giving out a move action a few times per day, a crusader can give out an entire turn once per fight (White Raven tactics).
I see from comments that some members of the party are “iffy” on Tome of Battle. They shouldn’t be—it’s easily the best-designed book in all of D&D 3.5e, and its classes are extremely tightly balanced with one another and fall squarely in the lower-middle range of 3.5e classes’ power levels. Each of the other members of the party is playing a class potentially much stronger than anything available in Tome of Battle.
Furthermore, our use of the book is going to be a lot of support and utility anyway. You shouldn’t be stealing the spotlight—you should be aiming the spotlight at your allies.
Now, White Raven is better with martial allies, there’s no doubt about that—but that’s true of the marshal, too. But nothing locks you in to White Raven as a crusader—you also get Devoted Spirit and Stone Dragon to play with, and both are great. Those will go a long way towards improving your battlefield presence, whether it’s through buffing or your own actions. Focus on White Raven—it’s on-brand, it’s better for your party’s concerns, and it’s strong—but grabbing stuff like Devoted Spirit’s healing maneuvers, or the mountain hammer line from Stone Dragon, can give you a ton of extra utility. (Note that mountain hammer ignores hardness, making it great for smashing through doors—or walls.)
And best of all, it qualifies you for Song of the White Raven, and with that feat, your crusader levels stack with bard for determining how strong your inspire courage is (not, sadly, how many times per day you can use it, so you’ll still want Extra Music).
It is generally advantageous to take your first crusader level later, since your non-crusader levels apply half to your crusader initiator level (this is also true, by the way, for the swordsage that we discussed for AC). Which works out nicely, since there are some non-crusader levels we want to take.
Human. You want a bunch of feats, and human gets you an extra one. Moreover, the only ability score you really care about is Charisma—and racial Charisma bonuses are rare and almost-always come with a Constitution penalty, which is bad. If you want to be a venerable Dragonwrought kobold (Races of the Dragon), or a lesser aasimar (Player’s Guide to Faerûn), those races would also certainly be powerful for you, but those are too cheesy for most games.
Notably, there is a human subrace known as silverbrow humans (Dragon Magic), which drop the extra skill point in favor of the dragonblood subtype (and some other minor things). This is nice just because dragonblood is required for Dragonfire Inspiration—not your top feat priority, but certainly a feat you’d very much like to have eventually, and having to take another feat—Dragontouched—to qualify is painful. I suppose you could dip dragon shaman (Player’s Handbook II) or dragonfire adept (Dragon Magic) to get more auras while you meet that qualification, but I don’t consider either worth it.
I strongly recommend that you go with a mishmash here, because you want several things available in the first couple of levels of several classes, and they synergize extremely well to accomplish the kind of martial buffing you’re interested in.
A key, but controversial, component here, I think, is having your 1st level be bard. Bard has the advantage of 6+Int skills—so you get the most out of quadrupling them for your 1st level—and qualifies you to take the 1st-level-only Precocious Apprentice feat (Complete Arcane pg. 181, not with the rest of the feats). That gets you a 2nd-level spell, which, since you’re a bard, means you meet Ascetic Mage’s requirement of being able to cast a 2nd-level arcane spell spontaneously (nothing in the feat says it counts as prepared). Then two levels of ranger or swordsage get us Charisma to AC in light armor, which is awesome.
I favor swordsage because maneuvers are great, but feel free to go with ranger if you prefer the BAB. Note that, like the crusader, it’s best to delay swordsage levels, so even though we’re taking bard 1st in order to be prepared to take Ascetic Mage, it’s probably better to hold off on that until about 6th level.
If you can’t do bard-first Precocious Apprentice, or don’t want to, Ascetic Mage’s 2nd-level spell requirement looks too painful for us, so we should probably skip it. Charisma to AC means battledancer, or we just ignore that and just wear armor—honestly, it should be fine. Ranger or swordsage are probably too far out of our way now, so battledancer can come in whenever if we want it.
Either way, after the one level of bard, your next priorities are marshal 2nd or 3rd, and paladin 2nd. If you can be a harmonious knight, and aren’t doing Ascetic Mage, you can even skip bard altogether in favor of just going straight to marshal and paladin—your skill points suffer, but a level is a huge thing. This also makes it more reasonable to take marshal out as far as 5th if you really want to, though again, grant move action is a lot worse than White Raven tactics, particularly for your magic-heavy party, and it’s somewhat doubtful that your second-favorite major aura will be that valuable.
And after that, just go all-in on crusader.
The two approaches work out to something like one of these:
2nd-level harmonious knight/2nd-level marshal/1st-level battledancer/14th-level crusader
You want to start with harmonious knight paladin just to qualify for Snowflake Wardance, Extra Music, and Dragonfire Inspiration for your human bonus feat, 1st-level feat, and 3rd-level feat. Then you can take Song of the White Raven at 6th.
Your other feats are wide open (and honestly, Dragonfire Inspiration is pretty optional). A couple Draconic Auras and then Double Draconic Aura at 12th might be on-brand though.
2nd-level harmonious knight/5th-level marshal/1st-level battledancer/11th-level crusader
- Basically the same as the above, but taking more marshal levels, and delaying crusader (and Song of the White Raven) until 9th. The advantage of taking crusader at 9th is that you get 3rd-level maneuvers and stances from your initial set. This is not, in my opinion, worth getting through levels 6th–8th without maneuvers, or Song of the White Raven, but if you wanted more marshal this might be the way to get it (if I were starting at 9th or higher, I might consider this option superior).
3rd-level bard/2nd-level paladin/2nd-level marshal/1st-level battledancer/11th-level crusader
Basically, a version of the above for games where harmonious knight is ruled not to work for bardic music feats.
Bard 3rd allows access to inspirational boost and Song of the Heart. Given you qualify, Song of the Heart is definitely a higher priority than Dragonfire Inspiration.
Could go with just bard 1st, and get more marshal levels or get into crusader earlier (probably at 6th so you can take Song of the White Raven then, and then pick up battledancer after).
1st-level bard/3rd-level marshal/2nd-level paladin/2nd-level swordsage/11th-level crusader.
Either your 1st-level feat or your human bonus feat has to be Precocious Apprentice for this to work.
You basically need to decide which of crusader and Song of the White Raven, and swordsage and Ascetic Mage, is going to happen at 6th, and which at 9th. Note that in both cases, you’ll need to save a level of marshal for after 6th, to make sure you qualify for the feat you want to get at 6th. The one that happens at 6th, obviously, happens sooner, and you’ll be able to enjoy that effect sooner. The one that happens later, you’ll get to choose 3rd-level maneuvers and stances for, which is a big deal.
I favor swordsage and Ascetic Mage for 9th level for two reasons:
Song of the White Raven is much more painful to be missing than Ascetic Mage. Without Ascetic Mage, you can still use light armor and be fine; the AC bonus is just that, a bonus. Without Song of the White Raven, you’re losing your first and most important turn in each and every combat.
You generally plan on leveling more of crusader, so you’ll have plenty of opportunities to get higher-level maneuvers and stances at higher levels. You’re only taking two swordsage levels, so you only get that one shot.
Taking swordsage 1st at 9th—which is what I recommend here—does mean taking Ascetic Mage before getting the swordsage’s AC bonus. That’s fine; Ascetic Mage only requires Improved Unarmed Strike, and will apply to the AC bonus once you get it.
All of these builds want as much Charisma as humanly possible, because they add Charisma to attack, AC, and all saves, as well as to the initiatives of everyone in their party. Both allow for powerful buffing in the form of auras and bardic music, get decent skill point totals, and allow you to be a respectable combatant in your own right (at least once crusader comes online). The former build is simpler, has more of the marshal class, and allows you to switch that Charisma bonus to the party’s initiative to more things. It also has more feats to play with at higher levels. The second build can wear light armor, has more maneuvers, and therefore should have more of a combat presence, particularly in the 6th–8th level range.
Neither build is going to keep up with mid-to-high-level spells. If the rest of your party plays those classes to the hilt, you will be outclassed in the back half of the game, and the most meaningful thing you’ll be able to do is make sure everyone on your side gets to go first with motivate Dexterity, and then White Raven tactics to give one of the spellcasters another turn.
If you are playing with multiclass XP penalties, you should stop doing that, because they’re terrible rules—but if you can’t, you’re basically limited to marshal 2nd in either build. Taking crusader earlier and for more levels is the obvious adaptation, but a there are other dips you could take instead of those levels that could also work well. I’m not going to bother delving into those unless requested, though, because almost no one plays with those penalties and no one should.