About where you stand
Duskblades should not multiclass
Duskblades get basically three notable things:
Spellcasting
The ability to attach a touch-attack spell to a weapon attack at 3rd level
The ability to apply that touch-attack spell to all attacks in a full-attack, at 13th level
Spellcasters never want to multiclass, because if they do, they lose out on their spells. If you got to, say, 5th-level duskblade/5th-level fighter, your 1st-level duskblade spells would be nearly pointless, which makes your five levels of duskblade pretty much a waste. You need to continue your duskblade spellcasting in order for duskblade to be worth anything. (Yes, this means that the cleric/scout and druid/sorcerer made severe errors in their choice to multiclass, though cleric, druid, and sorcerer are so high-power it might work out.)
The only exception someone could maybe try is to have three levels of duskblade, so you get arcane channeling, and then multiclass to some other, better arcane spell casting class, like wizard. A 3rd-level duskblade/5th-level wizard has 3rd-level wizard spells—while an 8th-level duskblade does not have 3rd-level duskblade spells. But really, this is very awkward—wizard obviously lacks much in the way of significant martial abilities, and adding a weapon attack to some wizard spells isn’t (remotely) worth three lost wizard levels. At that point, you should just be a wizard. Anyway, not an option for you.
The other possibility, but mostly a waste of duskblade and better done with a higher-power spellcaster like wizard, is multiclassing for a single level of Tome of Battle’s crusader, just so you could take the jade phoenix mage prestige class (discussed somewhat more below).
And since you indicate that your group is actually using the awful XP penalty rules—which is rather unusual—and that means you really do not want to multiclass, nixing either of those possibilities. For that matter, thanks to the XP penalties, no one should multiclass here (hopefully the cleric/archer and druid/sorcerer at least have one of those classes as favored class).
Seriously, consider asking your DM to ignore those rules. They’re horrid, and they make DMing far harder. Split-level parties are nothing but headaches, and the XP penalty rules do not offer anything good in exchange. The idea behind them (to punish multiclassing) is a bad one, and in any event, they don’t even do it that well. With those gone, you could maybe consider the crusader/jade phoenix mage route.
Prestige classes could be a good choice however
Unlike multiclassing to another base class, a prestige class can advance your spellcasting, making prestige classes a viable option. And on top of that, prestige classes do not apply an XP penalty.
However, few spellcasting classes grant perfect spellcasting advancement and full BAB—which duskblade itself does. In fact, the only option you really have is the abjurant champion from Complete Mage—which is a great class, but more for a full-spellcaster than for a duskblade. Still, it has a d10 HD, and you qualify without even trying, so that’s nice.
The big question is whether or not you will get to 13th level and the full-attack version of arcane channeling. At 12th level, a 7th-level duskblade/5th-level abjurant champion is better than a 12th-level duskblade. At 13th level, a 13th-level duskblade is vastly superior to an 8th-level duskblade/5th-level abjurant champion. So you have to decide, now probably, whether or not you think that this campaign is going to get to 13th and then see significantly more adventuring at that point.
Other than abjurant champion, there aren’t really a lot of great options:
Duskblades don’t get haste, so swiftblade is out.
XP penalties make multiclassing to Tome of Battle’s crusader in order to use the jade phoenix mage prestige class from that book rather punitive. You could do jade phoenix mage just by taking two Martial Study feats and a Martial Stance feat (both from Tome of Battle again), but three feats is an incredibly steep price, and Martial Study works far less well than a proper initiating class.
Anyway, if XP penalties were waived, jade phoenix mage is pretty good... but far better with stronger spellcasting. The d6 HD is obviously quite contrary to your goals, and since you cannot combine arcane channeling with a martial strike, duskblade and jade phoenix mage are fighting each other more than they’re synergizing.
You could really use to ret-con some choices, if allowed
Your Constitution and Intelligence are too low—and your Strength and Dexterity are too high. You would be better off focusing on either Strength or Dexterity. Considering the elven thinblade, Dexterity is the superior option. If you could get a feycraft one (Dungeon Master’s Guide II), that would allow you to use it with Dexterity without having to burn a feat on Weapon Finesse. Or you could just take Weapon Finesse.
Or go with Strength, which wants heavier armor but it’s not a huge problem, and use a weapon in two hands. Your AC will suffer, but AC, particularly armored AC, is the weakest defense in the game, and your damage will improve dramatically. Power Attack becomes critical in this case.
Going back to Tome of Battle, if you went the Dexterity-only route, you could take Shadow Blade to get Dexterity-to-damage with Shadow Hand weapons. Since the short sword is a Shadow Hand weapon, the thinblade ought to qualify. Shadow Blade also counts as Weapon Finesse for prerequisites, so you could even use feycraft to avoid taking Weapon Finesse, but still count as having it if anything else you wanted needed it.
Moreover, that thinblade is a really poor use of a feat. You could just use a rapier—also an elven weapon—and be almost exactly the same. The d8 damage die of the thinblade is a mere +1 average damage over the d6 of a rapier—a feat should get you way more than +1 damage. If you had Shadow Blade, the question would be somewhat harder—a rapier is not a Shadow Blade weapon—but then I would really just urge you to use a short sword rather than a rapier or thinblade. Elves don’t get automatic proficiency with a short sword, but that’s easily explained by a raptoran needing something smaller than the traditional longsword or rapier to fight while flying. It’s not like elves have a problem with short swords. Or, if you go the Strength route, you could just use a longsword—a greatsword would be somewhat better but the difference is minimal. (Keeping Exotic Weapon Proficiency, but going for the elven courtblade, would still be a poor use of a feat.)
And finally, Combat Expertise is another awful feat. Better than thinblade proficiency, but it is rarely the right choice to use it—and even though sometimes it is, a feat you are using rarely is not pulling its weight. If you were angling towards, say, Improved Trip, which requires Combat Expertise, that would be something, but then you should be using a reach weapon—spiked chains are Shadow Hand weapons—and Combat Reflexes.
As for how to replace these, in the Dexterity route, a Martial Study—aiming towards Shadow Blade—would be ideal, while in the Strength version, Power Attack is crucial.
Note that Player’s Handbook II has retraining rules, and there’s also the psychic reformation psionic power. These can help you with the feats, though not your ability scores.
What you should do going forward
Until 13th level, duskblade is very awkward as you have a good reason to not full-attack. Two-weapon fighting is out, charging with or without pounce is out, and that eliminates most of the best ways to increase your damage.
Flyby Attack
The Flyby Attack feat is excellent, though, so you should definitely take that at 6th. It allows you to perform any standard action—like arcane channeling—in the middle of your movement. That should do a lot to help your staying upright. It forces melee enemies, or even short-range ranged enemies, to chase you, or find another target. Make sure your ranks in Tumble are maxed so you can avoid attacks of opportunity.
Trip-lockdown: Reflexes and reach
I think the best way to take advantage of your ability scores, shore up your defenses ASAP, and avoid the problems of waiting until 13th level for full-attacks is to go for a trip-lockdown build. Unfortunately, this is not going to work with a thinblade—this will require either accepting that feat just being burned—which is awful—or some kind of ret-con, retraining, or psychic reformation. (Technically, a level of Tome of Battle’s warblade for the weapon aptitude feature would allow you to switch your Exotic Weapon Proficiency feat to something more useful, no retraining needed, and warblade is a pretty good class, but the XP penalties make this a no-go.)
If you really cannot do anything about the thinblade proficiency... I still think this is probably the best bet. You just have a wasted feat, which sucks, a lot, but I still think this works better than anything else.
As for what weapon you should use, a guisarme, the aforementioned spiked chain, or to get really fancy, a meteor hammer from Dragon vol. 319, would be awesome. Combine these with Combat Reflexes and Improved Trip, and you can mess with a lot of creatures. And even better, this actually uses both your Strength (for your Trip check) and your Dexterity (for your attacks of opportunity). Unfortunately, a lot of things are immune to tripping (most importantly, magical flyers) or just don’t really care about being prone.
Many of HeyICanChan’s suggestions for a tank fighter are very good options for you, too, if you go this route. Especially the feats that allow you to make attacks of opportunity against things that wouldn’t ordinarily provoke—Supernatural Instincts (Fiendish Codex II: Tyrants of the Nine Hells), Occult Opportunist (Dragon vol. 340), Martial Stance (thicket of blades) (Tome of Battle), even Complete Arcane’s Mage Slayer could be worth it, despite the −2 penalty to caster level. (If you really wanted, Practiced Spellcaster from Complete Arcane could solve that, but it’s probably better to just accept the penalty.) Knock-Down and Stand Still are also good suggestions for you.
Note also that spells like chill touch that allow for multiple touches can be cast with arcane channeling, and then should stay on your weapon for your following attacks—without spending more spell slots or actions on arcane channeling. Sandstorm has a particularly good one in parching touch, but that book did not consider duskblades—you’ll have to ask your DM if it should be on the duskblade spell list (I certainly think it should, for what it’s worth).
Anyway, with such a spell, you can use Flyby Attack to make an arcane channeling attack, casting chill touch or what have you, and then continue moving to your ideal position for tripping. The spell will linger on your weapon, allowing you to trigger it with your touch attack to trip, and then again with your follow-up attack from Improved Trip. And you already have Combat Expertise. Three hits of chill touch may well be better than a 3rd-or-4th-level spell.
Alternatively, planning for full-attacks: dual-wielding and swift mobility
Alternatively, particularly if you insist on using a thinblade, I would probably just start building with an eye towards full-attacks at 13th level. Until you get that ability, you have to choose between a full-attack or arcane channeling, and more and more the full-attack will be superior and your arcane channeling and spellcasting will be pointless—you’d be better off as a fighter. I would save abjurant champion until after 13th level, but I definitely would take it then.
Anyway, building towards full-attacks here probably means two-weapon fighting—charging would require you to multiclass and eat XP penalties, and it’s just not worth it. The gloves of the balanced hand from Magic Item Compendium can give you Two-Weapon Fighting, or Improved Two-Weapon Fighting if you already have Two-Weapon Fighting. That’s pretty valuable.
Multi-touch spells like chill touch continue to be a pretty good option here— they can allow you to make some ensorcelled full-attacks, on rounds after a standard-action arcane channeling, prior to 13th.
A single-level dip in cleric for the Travel Devotion feat (see Complete Champion for the feat and the rules for getting it as a bonus feat as a cleric) becomes a really good idea, since it allows you to move and full-attack, as your movement becomes a swift-action thing. And since cleric is a favored class for raptorans, no XP penalty. So you probably want Two-Weapon Fighting as your 9th level feat, and then probably squeeze cleric in at 14th before starting abjurant champion. Unfortunately, at that point Flyby Attack becomes much less useful—you again might ask after the retraining rules from Player’s Handbook II or psychic reformation here.
Capitalizing on spell repeats
Another feat to consider here is Poison Spell from Drow of the Underdark. Generally, poisons are pretty weak and expensive, though Craft (poisonmaking) (which has special rules in Drow of the Underdark and can let you make poisons for ⅙ their cost) can help with the latter. The reason Poison Spell is interesting is because it applies a single dose of a poison to a touch-attack spell—which means you can poison a spell and then apply that spell—and its poison—repeatedly with your full-attack arcane channeling. Poison Spell does risk poisoning you, as with poisoning a weapon, but you could take Master of Poisons (Drow of the Underdark again) to avoid that, and also get swift-action weapon poisoning. If you poison both your weapon and your spell, that would be double-poison on the first hit, which could be good (but conflicts with Travel Devotion, so only useful when you don’t have to move). The Arsenic & Old Lace Handbook is your go-to place for information on making the most of poisons.
Poison Spell again works with chill touch and the like, for before 13th or for the reach-and-trip version.
