With all the buffs we can exceed 20,000 average damage, but let's start at the beginning:
With no buffs
1,043.75 average damage [1,709 max]
Bugbear Gunner
Our archer for the no buff, no magic version is a Bugbear Fighter (Battle Master) 12/Rogue (Assassin) 5/Ranger (Gloom Stalker) 3 with a hunting rifle and purple worm poison on each bullet. Our allies are an 14th level Fighter (Battle Master), a 7th level Paladin (Oath of the Sentinel), a 6th level Warlock (The Fiend), and a 2nd level Wizard (Chronurgy Magic). This provides the following damage elements:
- 4 attacks per action via Extra Attack and Dread Ambusher.
- Action Surge to take the Attack action again.
- 1 attack via Commander's Strike
- These 9 shots are possible since we can reload our hunting rifle once with our bonus action.
- 5 ASIs for the Athlete, Gunner, Martial Adept, Piercer, and Sharpshooter feats
- 7 d10 Superiority Dice (5 from levels, 1 from Superior Technique, 1 from Martial Adept)
- 1d12 extra from the Commander's Strike
- 2 Sneak Attacks (3d6 each)
- 2 Dread Ambusher Attacks (1d6 each)
- 9 Surprise Attacks (2d6 each); our Paladin, Warlock, and Wizard help us go first
- Every attack is a critical hit from Assassinate
- One die gets rerolled with Piercer
- Purple Worm Poison damage (doesn't benefit from the critical hit)
Altogether, our average damage is as follows:
$$9*((1*6.75)+(4*5.5)+(4*3.5)+(12*3.5)+10+5)+7*(2*5.5)+2*(6*3.5+2*3.5)+1*(2*6.5) = 1043.75$$
With magic items and boons
1,889.75 average damage [3,203 max]
The Oathbow
The previous build essentially works as is with one deviation: we replace our hunter rifle with an oathbow, use arrows of slaying and attune to the Book of Exalted Deeds and the Eye of Vecna for 3 major beneficial properties of 1d6 extra damage each. This changes our base damage from our weapon from 2d10 to 6d10 + 1d8 + 3d6 and gives 3d6 extra damage per attack.
As such, here is our average damage:
$$9*((1*6.75)+(11*5.5)+(3*4.5)+(6*3.5)+(6*3.5)+(4*3.5)+(12*3.5)+10+5)+7*(2*5.5)+2*(6*3.5+2*3.5)+1*(2*6.5) = 1889.75$$
With the buffs
23,668 average damage [40,768 max]
The Characters
If we are able to use buffs, a different build is in order. We should now be a Duergar and here are the classes of our new build:
- Fighter (Eldritch Knight) 3 ---------------------------------------------------------- for Superior Technique, Action Surge, and the wizard spell list
- Rogue (Assassin) 17 --------------------------------------------- for Assassinate, the Martial Adept, Metamagic Adept, Piercer, and Sharpshooter feats, and Death Strike
...here are our magic weapons:
- longbow
- Book of Exalted Deeds
- Eye of Vecna
- Hand of Vecna
- sword of the paruns
- 14 arrows of slaying
- potion of giant size
- 3 spell scrolls of shapechange
...and here are our allies:
- 14x Clerics (Grave Domain) 2 ---------------------------------------- for Path to the Grave
- Bard (College of Valor) 18 -------------------------------------------- for Combat Inspiration, the spells wish and simulacrum via Magical Secrets
- Bard (College of Lore) 18/Fighter 2 ---------------------------------- for glyph of warding, the spells banishing smite, branding smite, elemental weapon, flame arrows, simulacrum, spirit shroud, and wish via Magical Secrets, Action Surge, and the Boon of High Magic
- Bard (College of Lore) 18/Fighter 2 ---------------------------------- for glyph of warding, the spells divine favor, enlarge/reduce, haste, holy weapon, simulacrum, Tenser's transformation, and wish via Magical Secrets, Action Surge, and the Boon of High Magic
- Fighter (Eldritch Knight) 17 ------------------------------------------- for Action Surge x2, the wizard spell list, the Metamagic Adept feat, and Combat Inspiration
- Bard (College of Valor) 15 --------------------------------------------- for Combat Inspiration
- Fighter (Eldritch Knight) 17 ------------------------------------------- for Action Surge x2, the wizard spell list, and the Metamagic Adept feat
- Bard (College of Valor) 15 --------------------------------------------- for Combat Inspiration
- Bard (College of Valor) 15/Cleric (Order Domain) 1/Fighter 2 --- for Combat Inspiration, Voice of Authority, and Action Surge
- Bard (College of Valor) 15 --------------------------------------------- for Combat Inspiration
- Fighter (Battle Master) 10/Bard (College of Valor) 10 ------------- for Commander's Strike and Combat Inspiration
- Bard (College of Valor) 15 --------------------------------------------- for Combat Inspiration
- Fighter 6 ----------------------------------------------------------------- for the Squire of Solamnia and Knight of the Crown feats
The Procedure
- The first Grave Domain cleric uses Path to the Grave on the target.
- The 13 other Grave Domain cleric Readies Path to the Grave for after our Duergar attacks.
- Our 18th level Valor Bard gives Bardic Inspiration to our Duergar
- Our 18th level Valor Bard then casts wish replicating the effects of simulacrum targeting one of the Lore Bards.
- Each new simulacrum casts wish replicating the effects of glyph of warding to store
- Each new simulacrum then uses Action Surge to cast wish replicating the effects of simulacrum targeting one of the Lore Bards
- This is done until we have 35 spell glyphs with the following spells: 14x banishing smites, 14x 8th level branding smites, 7th level elemental weapon, 7th level spirit shroud, divine favor, enlarge, haste, holy weapon, and Tenser's transformation
- The final simulacrum uses Action Surge to cast flame arrows at 5th level on our Duergar's quiver
- One of the Lore Bards administers the potion of giant strength to our Duergar
- Our Duergar uses a spell scroll of shapechange with the Quicken Spell Metamagic to turn into a Marilith and then takes the Attack action three times
- Both Eldritch Knights use a spell scroll of shapechange with the Quicken Spell Metamagic to turn into either a duergar warlord, or a duergar mind master
- Both Eldritch Knights then use Call to Attack or Mind Mastery respectively
- Both Eldritch Knights then use Action Surge to Ready those same actions for the following turns
- On those proceeding turns, two of our 15th level Valor Bards give Bardic Inspiration to our Duergar before the Eldritch Knight actions
- Our Order Domain cleric gives Bardic Inspiration to the Duergar and then casts a spell on the Duergar using Voice of Authority
- Our Order Domain cleric then uses Action Surge to Ready a spell for the purpose of Voice of Authority on the following turn
- On that proceeding turn, one of our 15th level Valor Bards give Bardic Inspiration to our Duergar before the Order Domain cleric action
- Our Battle Master gives Bardic Inspiration to our Duergar and then uses Commander's Strike
- Our final 15th level Valor Bard (who is attuned to a sword of the paruns) gives Bardic Inspiration to our Duergar and then uses the first property of the sword on the Duergar
- Our Knight of the Crown grants one last attack to our Duergar
The Calculation
Here are the damage elements with all the buffs:
- 2 base attacks from to Tenser's transformation; 2 more attacks from Action Surge; 1 more attack from haste
- 9 reaction attacks from 2x Call to Attack, 2x Mind Mastery, 2x Voice of Authority, Commander's Strike, sword of the paruns, and Knight of the Crown
- Each attack does a base damage of 3d8 + 6d10 + 5 with longbow, arrow of slaying, and potion of giant's strength
- Each attack has the following source of extra damage:
- branding smite for 5d10
- 8th level branding smite for 8d6
- 7th level elemental weapon for 3d4 + 3
- 7th level spirit shroud for 3d8
- divine favor for 1d4
- enlarge for 1d4
- holy weapon for 2d8
- Tenser's transformation for 2d12
- flame arrows for 1d6
- Major Beneficial artifact properties for 4d6
- Sharpshooter for 10
- 10 attacks benefit from Sneak Attack for 9d6
- 6 attacks benefit from 15th level Combat Inspiration for 1d12
- 2 attacks benefit from Superiority Dice for 1d6
- 1 attack benefits from the following sources of extra damage:
- 10th level Combat Inspiration for 1d10
- Commander's Strike for 1d10
- We reroll one damage die per attack from Piercer
- Every attack is a critical hit from Assassinate (and Piercer grants an extra d8)
- Damage per attack is quadrupled with Path to the Grave and Death Strike
- 14 doses of Purple Worm Poison
Here is the full calculation (dice stacked to take up less space):
$$4*(14*((1*8)+(3*6.5)+(22*5.5)+(17*4.5)+(26*3.5)+(10*2.5)+10+5+3)+10*(18*3.5)+6*(2*6.5)+2*(2*3.5)+1*(4*5.5))+14*(12*3.5) = 23668$$
Note: This relies pretty heavily on turn order. You can force the turns to take a specific order with sufficient quantity of Divination wizards using Portent