Build 1: Arc Lightning (adimixed fire) genasi Academiy Mage Infernal Prince fire-blaster +19 attack base, +.75 (from academy, +.75 from enervation v. reflex = 83% accuracy. 1d6+26 brutal 1 = 25.37, +3 if both targets hit = 54.82 fire and lightning DPR total to 2 targets. 3.28 round kill, which is acceptable, but not particularly good. Flame Dragontooth wand is better than an accurate staff of ruin this level, and there are no particularly compelling crit effects to go for incendiary daggers. Significant MAD.
Build 2: Arc Lightning (admixed fire) Tiefling Infernal prince Windrider with mark of storms. +21 attack base, 1d6+26 brutal 1 = 26.27 dpr, 56.84 DPR with slide 1.
Build 3 Winged Horde (admixed thunder) Human Primordial Adept Windwalker. +19+2 attack v. will, 1d4+21 = 21.77 dpr, 75.10 total DPR, at-will. slide 1.
Notes for Build 1
A 4e wizard pyromancer has many options. I don't misdoubt that you're having trouble building an "impressive" nuker.
At the bottom of this answer will be all the builds used to evaluate the suggestions up top. Step 1 is to build a genasi fire-blaster. The guide recommends genasi for non-fireblasters, so it's time to run the numbers for your at-will.
Stats are... remarkably tricky. You need int primary, str primary (Taking a page from the sorcerer handbook when you're adding your secondary stat, there are some strong arguments for even primary-secondary.)
But it gets worse: You need dex 13 (Dual implement spellcaster), Wis 13 (enlarge spell), and con 13 (pyromancy zone)
Now, presumably you're not going for genasi lightningblaster through mark of storm (which has its own MAD, but wind-rider is an excellent PP). Most of the PPs aim for the level 16 feature, so finding an appropriate level 11 thingy is an interesting challenge.
The default recommended PP for me is "Academy Master" simply because of its level 11 benefit that provides a (approximate .8 to hit at optimized levels of accuracy.) The choices change based on element, though. The master of flame PP only really "competes" (and then, not well) at level 16, though it does offer some fascinating synergies with winged horde. Theme of Infernal Prince for accuracy.
For build 1: Arc Lightning genasi fire-blaster +19 attack base, +.75 (from academy, +.75 from enervation v. reflex = 83% accuracy. 1d6+26 brutal 1 = 25.37, +3 if both targets hit = 54.82 DPR to the encounter as a whole. 3.28 round kill, which is acceptable, but not particularly good.
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level 13
Genasi, Mage
Level 1 Apprentice Mage: Pyromancy Apprentice
Level 4 Apprentice Mage: Evocation Apprentice
Level 5 Expert Mage: Pyromancy Expert
Level 8 Expert Mage: Evocation Expert
Master Mage: Evocation Master
Arcane Admixture Damage Type: Arcane Admixture Fire
Elemental Manifestation: Stormsoul
Arcane Admixture Power: Arc Lightning
FINAL ABILITY SCORES
Str 20, Con 11, Dex 14, Int 22, Wis 11, Cha 9.
STARTING ABILITY SCORES
Str 15, Con 10, Dex 13, Int 17, Wis 10, Cha 8.
AC: 22 Fort: 22 Reflex: 22 Will: 18
HP: 69 Surges: 6 Surge Value: 17
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +9, Diplomacy +7, Dungeoneering +6, Endurance +10, Heal +6, History +12, Insight +6, Intimidate +9, Nature +8, Perception +6, Religion +12, Stealth +8, Streetwise +5, Thievery +8, Athletics +11
FEATS
Level 1: Elemental Empowerment
Level 2: Superior Implement Training (Dragontooth wand)
Level 4: Dual Implement Spellcaster
Level 6: Destructive Wizardry
Level 8: Wand Expertise
Level 10: White Lotus Enervation
Level 11: Arcane Admixture
Level 12: Fiery Blood
POWERS
Level 1 Mage At-Will Powers: Arc Lightning
Level 2 Mage Utility Powers: Mystical Debris
ITEMS
Flame Dragontooth wand +3, Accurate orb of Nimble Thoughts +3
====== ======
Notes for Build 2:
Tieflings are all about fire buffing. Wind-rider gets us +1 to attack and +con to damage for lighting which synergizes with the +con zone bonus from pyromancy.
====== Created Using Wizards of the Coast D&D Character Builder ======
level 13
Tiefling, Mage, Lyrandar Wind-Rider
Level 1 Apprentice Mage: Pyromancy Apprentice
Level 4 Apprentice Mage: Evocation Apprentice
Level 5 Expert Mage: Evocation Expert
Level 8 Expert Mage: Pyromancy Expert
Master Mage: Pyromancy Master
Arcane Admixture Damage Type: Arcane Admixture Fire
Arcane Admixture Power: Arc Lightning
FINAL ABILITY SCORES
Str 9, Con 18, Dex 14, Int 22, Wis 11, Cha 13.
STARTING ABILITY SCORES
Str 8, Con 15, Dex 13, Int 17, Wis 10, Cha 10.
AC: 22 Fort: 20 Reflex: 22 Will: 19
HP: 76 Surges: 10 Surge Value: 19
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +13, Diplomacy +9, Dungeoneering +6, Endurance +12, Heal +6, History +12, Insight +6, Intimidate +11, Nature +6, Perception +6, Religion +12, Stealth +10, Streetwise +7, Thievery +8, Athletics +5
FEATS
Level 1: Mark of Storm
Level 2: Hellfire Blood
Level 4: Hellfire Arcanist
Level 6: Dual Implement Spellcaster
Level 8: Wand Expertise
Level 10: Superior Implement Training (Dragontooth wand)
Level 11: Arcane Admixture
Level 12: Secrets of Belial
POWERS
Level 1 Mage At-Will Powers: Arc Lightning
ITEMS
Flame Dragontooth wand +3, Accurate orb of Nimble Thoughts +3
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Notes for build 3
Winged horde is way too fiddly to go genasi, since we can't afford a huge str investement. The trick here is that we can spam a area burst 1+1+1 (3) party friendly zone of thundering psychic damage. (also, by going with the battleweaver PP, you can slow. All. enemies. With this. Your GM will throw dice at you.)
Going from my calculated AoE targeting maths, a area burst 3 targets approximately 3.45 targets throughout a battle. Combined with perma-CA through so many sources and an improved crit range from a bloody feat... gets us an effective +21 v. will // 1d4+19 psychic and thunder damage. (1d6 if you want to shrink the zone). 20.07 DPR per target... 69.24 total damage.
====== Created Using Wizards of the Coast D&D Character Builder ======
level 13
Human, Mage, Lyrandar Wind-Rider
Level 1 Apprentice Mage: Evocation Apprentice
Level 4 Apprentice Mage: Illusion Apprentice
Level 5 Expert Mage: Illusion Expert
Level 8 Expert Mage: Evocation Expert
Master Mage: Illusion Master
Arcane Admixture Damage Type: Arcane Admixture Thunder
Human Power Selection: Heroic Effort
Arcane Admixture Power: Winged Horde
FINAL ABILITY SCORES
Str 9, Con 16, Dex 14, Int 22, Wis 14, Cha 12.
STARTING ABILITY SCORES
Str 8, Con 13, Dex 13, Int 17, Wis 13, Cha 11.
AC: 22 Fort: 20 Reflex: 23 Will: 21
HP: 74 Surges: 9 Surge Value: 18
TRAINED SKILLS
Arcana +17
UNTRAINED SKILLS
Acrobatics +8, Bluff +9, Diplomacy +7, Dungeoneering +8, Endurance +11, Heal +8, History +12, Insight +8, Intimidate +9, Nature +8, Perception +8, Religion +12, Stealth +10, Streetwise +7, Thievery +8, Athletics +5
FEATS
Human: Echoes of Thunder
Level 1: Mark of Storm
Level 2: Staff Expertise
Level 4: Superior Implement Training (Mindwarp staff)
Level 6: Dual Implement Spellcaster
Level 8: Enlarge Spell
Level 10: Aggressive Advantage (retrained to Resounding Thunder at Level 12)
Level 11: Arcane Admixture
Level 12: Phantasmal Destruction
POWERS
Level 1 Mage At-Will Powers: Winged Horde
ITEMS
Resplendent Gloves (heroic tier), Headband of Intellect (heroic tier), Mindwarp staff of Ruin +3, Crystal orb of Nimble Thoughts +3
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I think you're right by RAW; elves' Fey Ancestry and bards' Countercharm only grant advantage on saving throws against being charmed, and therefore can't prevent something inevitable like this. There are a number of spells in the same boat, such as Aura of Purity.
Of course, there's a flip answer of "don't become incapacitated" and "don't let the warlock touch you". The rules are full of ways to do that, although some of them are complicated and most of them are fallible in one way or another. But, let's say that those things are unavoidable...
Prevention
Spells:
- Protection from Evil and Good (1st level cleric, paladin, warlock, and wizard spell) possibly could work, although it only prevents being charmed by aberrations, celestials, elementals, fey, fiends, and undead.
- Magic Circle (3rd level cleric, warlock, and wizard spell) is the same, but curiously leaves out aberrations.
- Hallow (5th level cleric spell) Has the same caveat. This has a 24-hour casting time and consumes 1000gp, but has a duration of "until dispelled", so if you know you're going to be fighting a thrall-creating warlock outsider, this might be a way to prepare.
- Contingency (6th level wizard spell) plus one of the cure spells of 5th level or lower (presumably Remove Curse, since that's the one available to wizards). This isn't quite prevention, since it can't keep the effect from taking hold for an instant before being removed, but I'm going to count it as such, because while it probably feels unpleasant, being charmed isn't that bad when no one has any time for actions.
- Antimagic Field (8th level, cleric and wizard spell) prevents magical charms from affecting targets in the sphere; presumably the "alien magic of your patron" is also temporarily suppressed when thralls are in the field, unless the patron counts as an "artifact or deity".
- Mind Blank (8th level bard and wizard spell) This is probably the winner. Lasts 24 hours (not concentration; whew!) and grants immunity to the charmed condition. It's 8th level, though, so the Create Thrall ability becomes available to players one level earlier.
- Wish (9th level sorcerer and wizard spell) One of the standard effects is immunity to a specific magical effect for 8 hours for up to 10 creatures; you could chose this one. But using Wish in this way risks adverse effects including possibly being unable to ever cast Wish again. If you just need to protect one creature and have access to Wish but not Mind Blank, using Wish to replicate the effect of that spell would be a better option.
Other:
Don't be humanoid. The effect specifically targets humanoids. As I read the rules for effects like Wild Shape and similar, it appears that these make you no longer humanoid, but it also does not seem to be spelled out exactly in the rules we have now. The effects do tend to say "the target's game statistics [...] are replaced by the statistics of the chosen [form]" — but it's kind of nebulous to me whether "creature type" is a game statistic in 5e, or whether affected creatures are "humanoids in beast form" for the purposes of this spell, or truly "beasts". In any case, the 8th-level spell Animal Shapes specifically says that it "turns others into beasts"; if making a DM judgment call, one might separate out the spells that preserve mental ability scores from those like Polymorph, which do not.
Be a high-level berserker. At 6th level, the Path of the Berserker barbarian gets Mindless Rage, which prevents being charmed while raging. A rage ends if the barbarian doesn't keep attacking (or taking damage), which is likely to happen if you're incapacitated. But the 15th level Persistent Rage removes that limitation, so as long as you stay conscious, you're safe — at least, until the 1 minute limit.
Cures
- Monk's Stillness of Mind, which can end a charm effect as an action
- Remove Curse (3rd level cleric, paladin, warlock, and wizard spell) by the ability's own description, of course.
- Dispel Evil and Good (5th level cleric and paladin spell) can end charms with a touch
- Greater Restoration (5th level bard, cleric and druid spell) ditto
- Hallow can also end the charm effect, with the same "outsider" caveat under "Prevention"
- Power Word Heal (9th level bard spell)
- Wish (by emulating Remove Curse)
Best Answer
To optimize DPS on a Warlock, you're looking to get as much Charisma as possible, and take the Eldritch Blast cantrip along with the Agonizing Blast invocation. Of the races you've said you'd prefer, the Half-Elf is the best for this. With your level 4 ability score increase, this gets you to 20 Charisma straight off the bat.
The next thing I'd recommend would be to take the Hex spell to add an extra d6 to your attacks. This gives you a d10 + d6 + 5 attack while your Hex is active, and since it's a level 1 spell that lasts up to an hour you should have it for every fight.
This gives you your basic attack. Outside of that, Shatter is your go-to spell for AoE damage (the Warlock doesn't have many options in this area). If you're looking to improve your hit chance rather than directly boosting your damage, replacing Hex with the Darkness spell and the Devil's Sight invocation will get you advantage against most enemies.
As far as feats are concerned, there aren't many options that will improve your damage. Spell Sniper would let you ignore half and three-quarter cover, which may come in handy, but its other features don't help you much. Crossbow Expert will let you use Eldritch Blast while in melee without taking disadvantage, but doesn't give you anything else you're interested in. War Caster will help you maintain concentration, but more importantly, it will allow you to make opportunity attacks with your Eldritch Blast. That said, in order to get opportunity attacks you need to be in melee, which won't work well without Crossbow Expert.
So Crossbow Expert combined with War Caster will give you a powerful oportunity attack, but it requires 2 feats and you can't rely on getting opportunity attacks often. Crossbow Expert is still useful on its own so that your attacks don't suffer when an enemy gets into melee with you, but overall, you might be better off going with Moderately Armoured as you've planned.