Any race published is allowed you can use the Customize Your Origin feature from Tasha’s Couldron of Everything.
Any officially published Class by WotC is allowed as well as Optional Class Features.
If there is a circumstantial bonus, such as Assassinate or Haste, please show each step in damage calculation.
No magic items, charms, blessings, or epic boons.
No, the "damage done" can’t be to yourself.
You are on your own no other PC or 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.
Consider your target to have AC25; Skill Checks str(+10), dex(0), con(+10), int(-4), wis(0), cha(0); Saving Throws Str +10, Dex +0, Con+10, Int +5, Wis +9, Cha +9;
Damage Immunities Fire, Poison; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks Condition Immunities Charmed, Frightened, Paralyzed, Poisoned Senses Blindsight 120 ft., Passive Perception 10
Since we are talking LV 20 for the purpous of overcomming immunity your weapons may count as magical
You may have one non-combat round of setup outsite the blindsight range of the Tarrasque. Longstride can be cast in advance