One-Rounding The Tarrasque: About Halfway There

[JNAProductions]

Okay. Just kinda spitballing here...

Bugbear Champion 20
Gear: Oathbow (Attuned), eight +3 Arrows, Bracers Of Archery (Attuned), Ioun Stone Of Mastery (Attuned)
Starting stats: 17 Dexterity, everything else irrelevant
Fighting Styles: Archery and Superior Technique
Maneuvers: Ambush, Precision, one of choice that adds damage on a hit
ASIs and Feats: Piercer (+1 Dexterity), Fey Touched (for Bless), +2 Dexterity, Martial Adept, Alert, Sharpshooter, Fighting Initiate (if Close Quarters Shooter is on the table) or one of choice (if not)

Round Zero-Preparations
Cast Bless. Wait for caster friend to teleport you into engagement range or otherwise get ready to initiate combat.

Round One-Halfway To Murder, About
Roll initiative. You have 1d20+10 versus 1d20, and can add +1d6 if need be. Going first is of paramount importance.
Declare the Tarrasque as your sworn enemy, gaining advantage on all attack rolls made with the Oathbow against it, and dealing an extra 3d6 damage on each hit.
Action Surge.
Shoot him eight times, for 2d20b1+17+1d4, plus 1d6 if you miss by a small amount up to three times (assuming one was not needed for initiative). Each hit does 1d8+5d6+10 (32) damage, each crit does 3d8+10d6+10 (58.5) damage. Reroll the lowest damage d8 with Piercer.
Sharpshooter adds 10 damage but drops hit bonus to only 12. It's the better choice, though.

Basic Attack
AC 25, which means we have a .9463 chance of hitting. We also have a .1413 chance of critting. So that's (.1413*58.5)+((.9463-.1413)*32)=34.02605 damage per attack.

Sharpshooter
AC 25 gives us a .7713 chance of hitting, same of a crit. So that's (.1413*68.5)+((.7713-.1413)*42)=36.13905 damage per attack. This is the better option.

With each attack dealing about 36 damage on average, we're looking at 288 damage in one turn. Unfortunately, this isn't replicable on turn two-Bugbear's Surprise Attack is lost then. The question is, then, how do we get enough damage to solo the Tarrasque in one turn?

Edit: Talking with an awesome lovely, who suggested switching to Fighter 17/Assassin 3 (for autocrits) meaning we can switch to Battlemaster for better Superiority dice. We've got eight Superiority dice with everything, meaning we can use one on every attack.

But, does Fighter 12-16 give anything worth it? Swap Fey Touched for Potion of Heroism, and we've got three ASIs free-lose one from Fighter 19, another from 16, another from 14 and one from 12. Which means we probably need Fighter 12 (that'd be three ASIs down, which is fine). That does leave 5 levels to futz with.

Gloomstalker 3! That leaves two levels left to mess with, but at level 20...

Battlemaster 12/Assassin 4/Gloomstalker 4
Gear: Oathbow (Attuned), nine +3 Arrows, Bracers Of Archery (Attuned), Ioun Stone Of Mastery (Attuned), Potion Of Heroism, Potion of Speed
Starting stats: 17 Dexterity, 14 or 15 Wisdom, everything else irrelevant
Fighting Styles: Archery and Superior Technique
Maneuvers: Ambush, Precision, one of choice that adds damage on a hit, and whatever else you want
ASIs and Feats: Piercer (+1 Dexterity), +2 Dexterity, Martial Adept, Alert, Sharpshooter, +2 Wisdom

Round -1
Drink Heroism

Round 0
Drink Speed

Round 1
Roll initiative. You have 1d20+13 versus 1d20, and can add 1d10 if you need to.
Declare the Tarrasque as your sworn enemy, gaining advantage on all attack rolls made with the Oathbow against it, and dealing an extra 3d6 damage on each hit.
Action Surge.
Shoot him nine times (four from each attack action, once from Haste action), for 2d20b1+17+1d4, plus 1d10 if you miss on anything but a 1. Each hit does 3d8+10d6+10, plus 2d10 if you didn't use Precision on the hit.
Reroll a d8 with Piercer, probably adding +3.5 damage.
Sneak Attack procs once, adding 4d6.

Though wait-we only have 5d10+2d6 Superiority dice, so not enough for EVERY attack.

Okay-if EVERYTHING hits, which is possible, we're looking at...

27d8+90d6+90+4d6+10d10+4d6, which is...
121.5+315+90+14+55+14
609.5

Add 3.5 for a d8 reroll, 613. Not enough.

Alright. I didn't want to have to do this, but...

Battlemaster 12/Assassin 4/Gloomstalker 4
Gear: Oathbow (Attuned), nine Arrows of Tarrasque Slaying, two Oils of Sharpness Bracers Of Archery (Attuned), Ioun Stone Of Mastery (Attuned), Potion Of Heroism, Potion of Speed
Starting stats: 17 Dexterity, 14 or 15 Wisdom, everything else irrelevant
Fighting Styles: Archery and Superior Technique
Maneuvers: Ambush, Precision, one of choice that adds damage on a hit, and whatever else you want
ASIs and Feats: Piercer (+1 Dexterity), +2 Dexterity, Martial Adept, Alert, Sharpshooter, Lucky

Round -21
Apply Oil of Sharpness

Round -1
Drink Heroism

Round 0
Drink Speed

Round 1
Roll initiative. You have 1d20+12 versus 1d20, and can add 1d10 if you need to.
Declare the Tarrasque as your sworn enemy, gaining advantage on all attack rolls made with the Oathbow against it, and dealing an extra 3d6 damage on each hit.
Action Surge.
Shoot him nine times (four from each attack action, once from Haste action), for 2d20b1+17+1d4. Each hit does 3d8+10d6+10, plus a DC 17 Con save against 6d10 (half on a pass).

We've got 99.92% chance of hitting him 6 times, and 91.91% chance of 8 times. (Thanks Anydice!)
All 9 hits will only happen 60.82% of the time, but a 1d10 Precision SHOULD solve that. But even with 8...

24d8 (weapon)+80d6 (Oathbow and Surprise Attack)+80 (static mods)+4d6 (Sneak Attack)+10d10 (Superiority Dice)+4d6 (other Superiority Dice) gets us to 551.
Eight arrows of slaying, when he passes on a 7+ (60% of the time) and has three Legendary Resistances means that we should get about two failed saves. So that's an additional 12d10+18d10, for 165.

Average of... 716 Damage!

That'll do!

[Eldariel]

That seems like it works. Quite the nova. Out of curiosity, what's the race? Seems like Bugbear would add a lot and potentially conserve some consumables. (NVM, realized it's in the first line) As an alternative, something simple:
Bugbear Bladesinger 17/Fighter 2/Hexblade 1

Lucky, Metamagic Adept: Subtle & Extend, +2 Int, +2 Int. Fighter takes like Superior Technique: Ambush.

Get Belt of Storm Giant Strength. Add Tome of Clear Thought. Make a Simulacrum and get them a Belt too (so your magic items are two Belts). On previous day, Extend Foresight on yourself. Shapechange into Marilith. Now you are a 29 Str Marilith. Then you start Bladesong and cast e.g. Teleport or Dimension Door or whatever to get to the Tarrasque. Obviously your Simulacrum does the same. Roll Initiative. Win. Initiate Hexblade's Curse. Your attacks now do 2d8+9+6+6+2d6 = 2d8+2d6+21 each (except tail, which is 2d10). You Action Surge for 14 of those. Your Simulacrum is the same except without advantage.


Overall probabilities:
96,38% to win Initiative (with Ambush, +5 Dex, Foresight and Lucky)
(93,60% to make the Wisdom-save vs. Frightful Presence (+7 from 12 Wis and prof, 2d20+Lucky)
84% for Simulacrum to make the Wisdom-save (+7 from 12 Wis and prof, Lucky))

Then your expected damage:
369 Swords + 65 Tails = 434 for yourself
241 Swords + 43 Tails = 284 for your Simulacrum

For a total of 718, which means you have fair margin of error (with 28 attack rolls of which 14 are at advantage meaning 42 d20 rolls, chances are extremely good that you're arriving close to the average) for...and extremely significant chance of one-rounding the creature. Sadly the damage math is involved enough that it would be too involved to calculate the exact percentage of dealing at least 671 but it should be way north of 90%.

EDIT: Hide of the Feral Guardian, Eldritch Claws Tattoo activation and a source of Bless (cast by Simulacrum or Potion of Heroism or whatever) and Haste (again, Simulacrum casting or Potion of Speed) would make for a solo kill without attacks from Simulacrum if we wanted to add that stipulation, filling out your Attunement while at it. Though without adding e.g. Potion of Growth, it'd be fairly barely (680) so reliability wouldn't be quite the same without Simulacrum also attacking.

Somewhat interestingly, if you let Big T attack with this item loadout, your AC is 18 + 3 + 6 = 27, and you have reaction on your own turn and enemy turn so Shield-spell vs. legendary action attack and Parry on enemy turn would mean AC 32 vs. normal attacks and AC 37 vs. Parry attack with Foresight giving Big T Disadvantage. So it would not be trivial for Big T to hit you either; AC 37 is basically automatic miss with Disadvantage and AC 32 is pretty rough too.

EDIT#2: I figure this is worth sharing even though it's ultimately a fail, Planetar Shapechange using Ascendant Dragon's Wrath Longbow (War Cleric 1 for bonus action attack) came up a bit short (a total of 530 damage). Used Bracers of Archery, said bow, Arrows of Slaying, Oil of Sharpness, Potion of Growth and the Bless/Haste setup. Can't really come up with a way to add sufficient amounts of damage outside Glyph of Warding abuse for like Spirit Shroud: the class levels are tight and melee weapon stipulations exist on most buffs. Assassin 3 over Fighter 2/Cleric 1 fails to get Action Surge (2 less attacks) meaning the overall damage is similar.

EDIT#3: Oh yeah, Bloodthirst Tattoo! Definitely a good third attunement. That's +4d6 on all the attacks on the Planetar Archer. Actually brings us fairly close: 620 with Potion of Growth in the mix. Switching to Hand Crossbow and adding a 3rd level for Battlemaster would add significant damage, to the tune of 640, but Arrow of Slaying doesn't exist in Bolt-form so that's probably not feasible. I wonder if there's a superior alternative to Bracers of Archery... Ring of Red Fury would add +6 instead of +2 so that puts us up to 640. Slaying Arrows are unlikely to come to our rescue here; 1% for Big T to fail 4+/6 saves (at least Silvery Barbs could be used for essentially an extra roll but that's unlikely to matter enough - 2,8% for failing 4/7). This is annoyingly close: it feels like there must be some way to put it over the top.

Few ways I considered:
- Go Fighter 3 for Battlemaster. Have Simulacrum attack but use Commander's Strike instead of actually attacking. This nets a Reaction attack which of course easily pushes the build over the top (at least if bonus action remains weaponized; e.g. Storm Boomerang from Quick Throw should suffice).
- Have Simulacrum Shapechange into a form that can buff allies (e.g. Hobgoblin Captain and use Leadership) but I couldn't think of a buff that doesn't require the Simulacrum to come along: an oldfashioned damage buff would be what the doctor ordered.
- Use a damaging spell of some kind but that feels off here since this is specifically about shooting it dead (and it's hard to find a spell that can deal as much damage as fully magicked out attack).
- Piercer, but it only adds like 4 DPR here since crits are only on 20.

...and then I realized I forgot to count the +3 damage from the ammunition. Adding that and Piercer (since there's a free feat after Lucky/MMA/Sharpshooter so why not) in puts the build at 661 damage, ~663 if we factor in the reroll too. Exactly 10 off. Honestly, if we Martial Adept for Precision Attack instead of Piercer, that probably adds enough but it's a pain to math out so I'm not going to.

EDIT#4: Simulacrum Shapechanging into Battleforce Angel lets it give the attacker +d4 on all attacks for one round without actually tagging along or needing to be present so that should work. Of course, it's from GGR, which is not an ideal source but hey, it gets the job done.