How to optimize existing PC for max level damage output

Question

I have a multiclass character that I'm interested in optimizing for endgame (level 20) damage output. The character in question is basically an archer-type built around crit-fishing and dumping massive damage on crits.

As such, he is a Drow Hexblade5/Gloomstalker4/Arcane Archer4. He has a 20 DEX, can Eldritch Smite, and has the Sharpshooter feat. As of now, the absolute best case scenario on a crit works out to be:

2d6(piercing) +2d4(Favored Foe) +2d8(Dread Ambusher) +2d6(necrotic from Hex) +8d8(3rd lvl Smite) +4d6(Shadow Arrow) +10(Sharpshooter) +14(other bonuses).

One downside is that this 'best case' has only happened like twice in 13 levels, but Holy Hells was it glorious when it popped off!! The other downside is warlock spell slots. I'm looking to either move away from crit-fishing to being able to Smite more often by taking 7 levels of wizard or cleric for more spell slots, or finding a way to dump an even bigger mountain of dice on crits.

My character has an unspent ASI that can be used to meet the prerequisites to be able to multi-class into any other published class. I had planned to use it for the Skill Expert Feat with WIS which would give him an 18 in that stat. Given the constraints of existing race/class/stat combos:

By 20th level, how can I either increase single crit damage output or gain higher sustained damage?

Answer 1

Three levels of Rogue (Assassin) would mean more effective novas on the first round of combat (especially if you achieve surprise). In particular, you'll get:

1. Once-per-turn +2d6 Sneak Attack damage (doubling to +4d6 when you crit)
2. Advantage on attacks against anyone who hasn't taken a turn in combat yet
3. Automatic crits against surprised enemies

It's not doing a huge amount for extra damage, but since you're clearly a little sad not to see that massive damage show up very often, this will mean that, on the first round of combat, if you surprise your foes, all your attacks will have advantage and crit if you hit, and one of your attacks will get a bonus +4d6 damage. It synergizes exceptionally well with the Gloomstalker's bonus first round attack, which in turn synergizes exceptionally well with Fighter's Action Surge, which can double Gloomstalker's first round extra attack with +1d8 bonus damage, for a total of six attacks, one with Sneak Attack damage, two with Gloomstalker bonus damage, and (with surprise) all of them doubling all their damage dice due to the auto-crits.

Even if you don't manage surprise, your high Dex means there will usually be some targets who go after you in the first round, and you'll get advantage attacking them, almost doubling your chance of critting (with Hexblade's Curse expanding your crit range, simply having advantage means a 19% chance to crit per attack, and with three opening attacks, you have ~47% chance for at least one of them to crit).

After that, I'd think you'd be best off putting more levels into Warlock. After three levels of Rogue, you'll only have four more levels to use, and four levels of Wizard or Cleric won't help much (just some low-level smiting slots). By contrast, bumping Warlock up to level 9 will bump your Pact Magic slots from level 3 to level 5, meaning smiting will gain you +6d8 (+12d8 if you wait to smite until you've got a crit) rather than just +4d8/+8d8. Pact Magic slots come back on a short rest, and you'll usually get to short rest every 1-2 encounters; sure it's not smiting every crit, but when you do smite, it will hurt a lot. Plus, two more Eldritch Invocations almost certainly have something to offer here (not knowing party composition or typical strategies, it's hard to make any concrete recommendations).

Oh, and just in case you haven't already figured this out: You should be burning your Hexblade's Curse and blowing your Action Surge in the first round of every combat if at all possible (get in those short rests folks!). Even now, that's worth six attacks, each with a 10% chance to crit, or 47% chance to crit in the first round. Once you've got Assassin 3, either they'll all be crits (if you get surprise and manage to hit; no need for Hexblade's Curse in that case), or if you at least go first, you'll get advantage, raising the per-attack crit chance to 19%, and the chance of at least one crit to nearly 72%. Gloomstalker+Action Surge makes for an insanely effective first round nova, even without smites involved.

Answer 2

Some Rules Things

I just wanted to clarify a couple rules item.

First, your perfect scenario involves two bonus action bonuses: Hexblade's Curse and the hex spell. You can certainly apply both of these by applying one outside of combat, but getting them both alongside the Gloom Stalker attack (which only applies on the first turn) can be difficult.

Remember that you only get one bonus action per turn, and that Dread Ambusher's extra damage only applies to the one attack extra that you get (emphasis mine):

If you take the Attack action on [your first turn of each combat], you can make one additional weapon attack as part of that action. If that attack hits, the target takes an extra 1d8 damage of the weapon's damage type.

Secondly, Eldritch Smite (unlike Divine Smite) requires specifically Warlock spell slots so multiclassing as a full caster won't give you more.

Now on to the optimization:

Advantage

The very first thing you should do is get a consistent source of advantage. This will help with the crit-fishing and sustained damage numbers. If you were hiding at the start of combat, only the first attack will have advantage, dramatically reducing the odds of a crit.

Right now (assuming you were hidden at the start of combat), you have a 52.2% chance to crit on the first round (assuming you used Action Surge), and a 19% chance to crit on later rounds.

If you instead get consistent advantage with your first action, these numbers change to 46.9% on the first round (giving up Action Surge attacks), but 34.4% on later rounds.

The best way to do this is to sacrifice hex (which was already hard to get because of the multiple bonus actions problem) in favor of shadow of moil which makes you an Unseen Attacker and therefore gives you advantage. This is accomplished with only two further levels of warlock and also improves your Eldritch Smite.

Since you are going to have advantage so often, get the Elven Accuracy feat for your free ASI. This will still get your Wisdom to 18 and improve your odds even more to 61.3% on the first round and 46.9% on later rounds.

Build So Far: Warlock (Hexblade) 7 / Ranger (Gloom Stalker) 4 / Fighter (Arcane Archer) 4

Now the builds diverge depending on whether you are interested in more crit-fishing bonuses, or sustained damage. I'll start with crit-fishing:

Too many crits, not enough smites

Since Eldritch Smite is only tied to warlock spells, the best way to get more smites (and more Shadow Arrows and Action Surges while you're at it) is to get access to the catnap spell. This will let you, once per day, use shadow of moil and one Eldritch Smite in two encounters before you need a full hour of rest.

The best option to get the spell help with our crits is Bard (College of Whispers) which gives just one more thing to add to our crits, Psychic Blades:

When you hit a creature with a weapon attack, you can expend one use of your Bardic Inspiration to deal an additional 2d6 psychic damage to that target.

You won't get catnap until level 20, but you will get Psychic Blades along the way as well as more spell slots to have fun options while you aren't smiting (these can be used with your warlock spells even if they can't be used to smite).

Final Build: Warlock (Hexblade) 7 / Ranger (Gloom Stalker) 4 / Fighter (Arcane Archer) 4 / Bard (College of Whispers) 5

I'm already napping

If your party already has an artificer, bard, sorcerer, or wizard who can cast catnap for your party, it is still good to grab Bard (College of Whispers) for three levels to add to your mountain of dice. However, those final two levels can be used elsewhere.

In this case, I'd recommend finalizing the build with two more levels of warlock to get your smite up to 5th level and gab the Crossbow Expert feat. This way, in combats with one major opponent that can soak up a lot of damage, you can use hold monster instead of shadow of moil.

Hold them with your Action Surge. Walk right up to the paralyzed foe with your Gloom Stalker speed. Get a guaranteed critical hit to dump your dice onto. This then let's all of your allies join into the crit-fishing game assuming they can fight within 5 feet of your target.

Final Build: Warlock (Hexblade) 9 / Ranger (Gloom Stalker) 4 / Fighter (Arcane Archer) 4 / Bard (College of Whispers) 3

Now for sustained damage:

Advantage always

If sustained damage is the name of the game, you really want advantage all the time to counteract Sharpshooter. This means your warlock spell slots are now for only shadow of moil. You then only have three first level smites from your ranger levels, but the advantage is more important than bigger smites for sustained damage.

Another important element is Concentration. While losing shadow of moil for a crit-fisher is somewhat okay once they've hit their big crits, it is not okay for sustained damage. I'd grab one more level in warlock to bump your Intelligence so you can multiclass into Artificer (which has a nice Infusion for Concentration).

First grab one more level of Warlock to get the ASI for 13 Intelligence; then take Artificer (Alchemist) for 4 levels. This gives you an alternative use of your low level spell slots than measley 2d8 smites. You can create Boldness Experimental Elixirs which function like the bless spell without the need for Concentration.

This also gives you access to Infusions like Enhanced Defense and Mind Sharpener to help with your Concentration until you get Resilient (Constitution) at level 20:

When the wearer fails a Constitution saving throw to maintain concentration on a spell, the wearer can use its reaction to expend 1 of the item's charges to succeed instead.

Final Build: Warlock (Hexblade) 8 / Ranger (Gloom Stalker) 4 / Fighter (Arcane Archer) 4 / Artificer (Alchemist) 4

Alternatively (and exclusively if your Intelligence is too low to reach 13 from one ASI), you could grab Resilient (Constitution) at level 16 and use your final 4 levels going even further into warlock.

This gets you an extra spell slot so that you have a further shadow of moil or a nice smite if you don't need three before a short rest. Additionally you can get access to a Mystic Arcanum (I recommend soul cage since your Charisma isn't that high for save DCs), some high level Invocations (including Lifedrinker when you hit 20th level), and a final ASI (probably to boost your Charisma for Lifedrinker).

Final Build: Warlock (Hexblade) 12 / Ranger (Gloom Stalker) 4 / Fighter (Arcane Archer) 4