Premise
Typically, you deal damage by hitting the other guy with a stick (or a fireball) [citation needed]; however, spells like fire shield damage enemies who hit you:
In addition, whenever a creature within 5 feet of you hits you with a melee attack, the shield erupts with flame. The attacker takes 2d8 fire damage from a warm shield, or 2d8 cold damage from a cold shield.
Similarly, hellish rebuke allows you to damage enemies in response to them damaging you. Either way, the enemy wouldn't have taken damage if they chose to just ignore you. This leads to my question: what is the highest average damage that a level 10 character can deal in response to being hit or damaged?
Restrictions
Yes, there are a lot of restrictions here. Most of them stem from the following desires:
- Answers should be comparable. Without restrictions, one build could look great, until you realize that it's built for one round of combat per long rest.
- Answers should resemble a playable character. Without restrictions, you end up with 30 Charisma, a slew of legendary items, and unintended UA feature interactions – fun, but not really practical.
- Answers should be relatively simple. I say "you never lose concentration" not because I'm using that rule (that'd be a terrible idea!), but because the alternative is much more complicated.
Character
- Level 10.
- May use the PHB and one of the following: EE, MToF, SCAG, VGtM, or XGtE. This restriction does not extend to spell selection; spells may come from any or all of those sources.
- No variant rules besides multiclassing, feats, and variant human.
- Up to three magic items from DMG Tables F or G with a maximum rarity of Rare.
Combat
The basics
- Four encounters; Combat 1, Combat 2, Short Rest, Combat 3, Combat 4 (each of these events is separated by 30 minutes). The Short Rest is mandatory.
- Each combat encounter has you face off against two earth elementals. These elementals do not have any damage vulnerabilities, damage immunities, or condition immunities. They keep their damage resistances.
- Each combat lasts three rounds. All participants get a turn in each round. Initiative order is up to you.
- The elementals use their Multiattack normally (moving into range, if needed). Compare the To Hit in the elemental's stat block to your AC as normal to determine how often the elemental hits. Hits deal average damage.
- For the sake of simplicity, treat enemies and the PC as if they had infinite non-temporary hit points.
- You never lose concentration as a result of taking damage.
The anti-cheese
Don't get me wrong – I like me some cheese, just not for every meal!
- Enemies do not make opportunity attacks.
- You may not use the Ready action. (Mainly to prevent "I ready blight for when I get damaged" from being the best answer.)
- Any spell or ability that lasts 10 minutes or longer may be activated before entering combat. Any spell or ability that lasts longer than 8 hours may be activated the day before. (Other spells may also be cast outside of combat, but will expire before combat begins.)
- You may not spend more than 100 gp on spells that consume costly material components. (So casting identify and find familiar is fine, because identify doesn't consume its costly component, but casting glyph of warding is not.)
- Allies cannot help you, unless you summon/create them yourself (via conjure animals, animate dead, etc.)
Damage calculation
- Damage should be the average damage per round over the course of the adventuring day.
- Only count damage dealt to enemies (the earth elementals) in direct response to, and in the same turn as, being hit or damaged. Hellish rebuke is fine. The extra damage dealt by absorb elements is not. You may still deal damage in other ways (in case it's necessary for setup), you just can't count it in the total.
Best Answer
To answer this effectively, we need to determine which spells are worth casting.
The three spells to really consider are Armor of Agathys, Fire Shield, and Shadow of Moil.
Armor of Agathys deals 25 damage per hit, but is limited - it only deals 50 total damage in this case before the temp hp runs out. We can try some ways to increase that further though, mostly by reducing damage taken from the elementals on each hit.
Fire Shield deals 9 damage per hit, is not limited, and is not concentration. Over the 12 attacks by enemies in one combat, this adds up to 108 damage if all hit.
Shadow of Moil deals 9 damage per hit, is not limited, and is concentration. This has the same overall damage output that Fire Shield does.
With that in mind, the best version is simply a Fiend Warlock 10 with two Rods of the Pact Keeper and a Ring of Spell Storing (Catnap). You'll also take the Alert feat, making you go first in ~75% of combats. You'll take the Lucky Feat to minimize critical strikes while Armor of Agathys is up. You choose the damage type of Bludgeoning for resistance as your Fiend 10 feature.
This gives you access to 8 spells over your four combats, though you still can't quite cast both Fire Shield and Shadow of Moil because of Rod logistics.
Combat 1
Precast Fire Shield
First round: If you go first, cast Shadow of Moil. If you do not go first, cast Armor of Agathys.
Second round: regain a spell slot with a Rod.
Third round: Cast Armor of Agathys
Catnap.
Combat 2
Precast Fire Shield
First round: If you go first, cast Shadow of Moil. If you do not go first and don't have temp hitpoints remaining from your previous cast of Armor of Agathys, cast it again.
Second round: Cast Armor of Agathys if possible.
Third round: Wait
Short Rest, change attunement to Rods.
Combat 3
Precast Fire Shield
First round: If you go first, cast Shadow of Moil. If you do not go first, cast Armor of Agathys.
Second round: Regain a spell slot with a Rod.
Third round: Wait
Combat 4
Precast Fire Shield
First round: Wait
Second round: Wait
Third round: Wait
You bring an AC of 10 and crawl into each combat, resulting in a hit chance of 99.75% for your enemies.
[108] - Fire Shield is responsible for 9 damage per attack over 12 attacks each combat. Cast 4 times.
[108] - Shadow of Moil is responsible for 9 damage per attack over 12 attacks each combat. This has a 73.75% chance of happening for each of the first three combats.
[100] - Armor of Agathys is responsible for 25 damage per attack over four attacks that each deal 7 damage. It is also relatively unaffected by hit chance. This has a 26.25% chance of happening for each of the first three combats, and one guaranteed cast.
430.92: Fire Shield total damage 432 * 99.75%
238.35: Shadow of Moil total damage 324 * 73.75% * 99.75%
178.75: Armor of Agathys total damage 300 * 26.25% + 100
848.02 total damage over 12 rounds of combat, or 70.66 damage per round.