What is the most (average) damage one character can do in one Round on its own to a Tarrasque?

Question

Rules/Restrictions:

Any race published is allowed. You may use the Customise Your Origin option from Tasha's Couldron of Everything. Any officially published Class by WotC is allowed as well as Optional Class Features. No Unearthed Arcana.

If there is a circumstantial bonus, such as Assassinate or Haste, please show each step in the damage calculation.

No magic items, charms, blessings, or epic boons.
Exception regular: +1, +2, +3 weapons

No, the "damage done" can't be to yourself.

You are on your own: there is no other PC nor an NPC.

Take into account that you might not surprise the Tarrasque.

You do not have advantage by default. If you can generate some advantage for yourself, then you may do so. Explain how you do it.

You are up against an Tarrasque. It will make use of its abilities including Legendary Resistance and Legendary Actions, if it has the chance to do so. All of its charges are full. Magic Resistance and Reflective Carapace work.

Link to the Monster Stats Block

Clarification: You can use any Race, Class, Spell or ability for the official published WotC books. Anything not listed in those book is not allowed. These are:

• Basic Rules (BR)
• System Reference Document (SRD)
• Player's Handbook (PHB)
• Sword Coast Adventurer's Guide (SCAG)
• Xanathar's Guide to Everything (XGtE)
• Guildmasters' Guide to Ravnica (GGtR)
• Mythic Odysseys of Theros (MOoT)
• Wayfinder's Guide to Eberron (WGtE)
• Eberron: Rising from the Last War (E:RftLW)
• Explorer's Guide to Wildemount (EGtW)
• Dungeon Master's Guide (DMG)
• Tasha's Couldron of Everything (TCoE)

If you have to roll a die for anything the outcome is random so statistic rules.

If you have the Longstrider Spell you may cast it in advance of the set-up round.

You start with one non-combat round for which you are more than 120 feet away from the Tarrasque. In this round you may, for example, buff yourself or summon other creatures with spells. But you can't enter the 120 feet radius around the Tarrasque. Initiative is rolled at the beginning of the combat round after the non-combat round. One full combat round occurs, so you can consider the Tarrasque attacking back if it can do so. It will use its Chomp with Swallow. For the Multiattack it will also use its Swallow action.

Answer 1

Expected Average Damage: 531.27, Maximum Damage:1248

This is my own go at the problem i asked to solve and the best i could come up with till now.

The Character

• Tabaxi (Pedro) (Feline Agility for extra speed to reach Tarrasque without dash action Stealth and Perception proficiency, Lucky feat from Mythic Odysseys of Theros character creation rules)
• Divine Soul Sorcerer 4 (Favoured by the Gods, Shadow Blade spell, Subtle Spell meta-magic, ASI)
• Circle of Spores Druid 2 (Halo of Spores, Symbiotic Entity)
• Assassin Rogue 3 (Assassinate - if surprised, advantage on all attacks, critical on all hits, 2d6 Sneak Attack with Finesse weapons per turn, Expertise: Stealth, Perception)
• Echo Knight Fighter 3 (Action Surge, Fighting Style Superior Technique, 1 Maneuver, Ambush, Manifest Echo, Unleash Incarnation)
• Oath of Conquest Paladin 3 (Divine Smite, Divine Sense, Fighting Style Blessed Warrior Guidance, Guided Strike)
• Gloom Stalker Ranger 5 (Dread Ambusher, Extra attack, Longstrider, ASI, Dueling, Favored Foe)

Pedros Abilities: {13/14/13/8/13/13}

With the Customise your Origin feature from TCoE he gets +2 to Dexterity and +1 to Constitution. Taking 2 ASI on the way and putting them all into Dexterity he ends up with {13/20/14/8/13/13}

• To-Hit: +11, +12-19
• Hit Points: 151 AC: 19 (Studded Leather, Shield) 20 AC is possible if you choose defending Fighting style
• Stealth: +16 (Expertise)
• Initiative: +9.5 +Lucky (+5 Dexterity, +1 Wisdom from Dread Ambusher 3+3,5 from Maneuver+ Lucky)
• Perception: +13 (Expertise) Level 9 Spellcaster (4 Sorcerer + 2 Druid
• 3 Paladin/2 + 5 Ranger/2)
• Movement speed: 30 feet (40 feet with Longstrider, First Round of combat 50 feet with Dread Ambusher)
• Darkvision: 90ft

The Combat

Subtle Longstrider beforehand. In the preparation round, cast Subtle Shadow Blade L5, use your Wild Shape (Symbiotic Entity) after which the combat round begins. Pedro has a +9,5 to initiative and three Lucky which gives him a 99.21% chance of going first.

Resource Expenditure:

Spending 1L5, 3L4, 3L3, 2L2 1L1 Spell Slots, all maneuvers, action surge, divine favor, Wild Shape (Symbiotic Entity), Manifest Echo, 2 Unleash Incarnation, 2 Sorcery Points

Order of Events:

Preparation Round: Stay 125 feet away from the Tarrasque

• Level 5 Shadow Blade (4d8 psychic damage per melee attack)
• Wild Shape (Symbiotic Entity) (1d6 necrotic damage per melee attack)

Surprise Round:

• Initiative: Lucky is used if needed the probability that Pedro goes first is:
  • 0 Lucky: 87.17%
  • 1 Lucky: 95.48%
  • 2 Lucky: 98.17%
  • 3 Lucky: 99,21%
• No Action: Activate Feline Agility and move 80 feet closer to the Tarrasque
• Bonus Action: Manifest Echo feet 15 away from you between you and the Tarrasque
• No Action: Move the Echo up to 30 more feet to get it into Melee range with the Tarrasque
• Attack Action: Make 2 attacks + 1 attack from the Dread Ambusher from the Echos location
• No Action: Use Unleash Incarnation with the Attack Action and make one more attack from the echos location
• Action Surge
• Attack Action: Make 2 attacks + 1 attack from the Dread Ambusher from the Echos location
• No Action: Use Unleash Incarnation with the Attack Action and make one more attack from the echos location
• No Action: Move 20 feet back to get out of range of the Tarrasque if he won the Initiative roll. Otherwise Move 20 feet in to keep you echo up for the next round.

The Math on the Damage

99.21% of the dice are doubled because these are critical hits because of Assassinate.

Every attack that hits results in this:

2×(4d8 Psychic (Shadow Blade, Level 5) + 1d6 Necrotic Symbiotic Entity + 2 Dueling Fighting Style + 5 Dexterity modifier) = 8d8 + 2d6 + 7 Psychic damage

8 attacks (4 attacks per action) yield: 64d8 + 56 Psychic damage + 16d6 Necrotic damage

Gloom Stalker Ranger allows us to adds 1d8 damage of the original weapon's damage type to the extra attack of every Attack action we take in the first round of combat through the Gloom Stalker feature. Since we take two Attack actions through Action Surge, we get extra 2d8 Psychic damage, which is doubled since they're critical hits. 68d8 + 56 Psychic damage + 16d6 Necrotic damage

Assassin Rogue allows us to add 2d6 Sneak Attack damage because Shadow Blade is a Finesse weapon. Since the attack is a critical hit, the damage gets doubled. 68d8 + 4d6 + 56 Psychic damage + 16d6 Necrotic damage

As a paladin, every successful hit allows us to add 2d8 + (spell slot level-1) * 1d8 radiant damage, maxing out at 5d8 for an L4 spell slot or higher. As a 9th level spellcaster, after casting Shadow Blade at level 5, we can use 3 L4 spell slots, 3 L3 spell slots and 2 L2 spell slots. All of this, of course, gets doubled on critical hits, which these will be. This results in the following additional damage:

2(3(5d8)+3*(4d8)+2*(3d8))= 66d8 Radiant damage**

Damage Total:

68d8 + 4d6 + 56 Psychic damage + 16d6 Necrotic damage + 66d8 Radiant damage = (684.5 + 43.5 + 56 Psychic)+(163.5 Necrotic) + (664.5 Radiant)= 376 Psychic +56 Necrotic + 297 Radiant = 729 total (Max: 1248)

Damage, adjusted for accuracy:

The following probability calculations below were made with the help of the website Clockworkmod.com

Pedro has Favored by the Gods, Guided Strike and Lucky to boost his accuracy and turn a miss into a hit.

With our target having an AC of 25, and our character having a starting +11 to hit, we need to roll 14 or higher in order to successfully land a hit, which is a 57.75% likelihood because we got advantage. When using Lucky the chance goes up to 72.54%. Favored by the gods brings one hit up to 83.375% but is best used if only a little bit is missing. Guided Strike works in 97.75% of the cases which is almost a guaranteed hit.

The good strategy could look something like this:

1. Our attacks, whenever possible, should be sequenced from most damaging to least damaging, in order to take advantage of our ability to lazily add attack bonuses.
2. Similarly, we should use our spell slots for Divine Smite in a decreasing progression. The first three attacks of the first Attack action will each use an L4 spell slot, the fourth Attack use an L3 spell slot. Two of the attacks from the second attack action will use a L3 spell slot and two more a L2 spell slot
3. If an attack misses and we still have lucky, we should use it giving us an extra d20 and increases the chance to 72.54 % to hit.
4. If the total for the attack roll is still too low and we still can use Favored by the Gods, we should use it.
5. Using Guided Strike if we roll between an 5 or a 6 or to guarantee the first hit sporting sneak attack and Dread Ambusher feature.

Using the tool mentioned above this gives the following expected damage:

• 0 Lucky 506,3
• 1 Lucky 509,7
• 2 Lucky 523,3
• 3 Lucky 535,7

Before we calculated on how many occasions we need to us our Lucky feat for Initiative.

This gives us:

506.3*(0.9921-0.9817)+509.7*(0.9817-0.9548)+523.3*(0.9548-0.8717)+535,7*0.8717= 5.27+13.71+43.49+466.97=529.44

Now we just need to add the number for the case where we don’t surprise the Tarrasque

We would expect 232.1 form that.

Which gives us:

529.44+232.1*(1-0.9921)=529.44+1.833=531.27

Result:

Adjusted for accuracy, then, this means that the expected average damage of this character is 531.27 which can spike way higher if we happen to roll well.