13th level ultimate one shot build (5e Optimized Character Build)
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Any, preferably human (Variant) for the free feat or Tiefling for a +2 to CHR (make sure to get the subrace with a +1 to DEX).
Strength: Little to none. Dexterity: Try to max this out as well. Constitution: Third stat you are gonna dump your points into. Intelligence: Little to none. Wisdom: Little to none. Charisma: Max this out
11 Levels in sorcerer and 2 levels in wizard It is extremely important that you get the Extended spell: Metamagic, Lore Mastery: Wizard subclass and Delayed Blast Fireball as a Sorcerer spell.
War Caster: You have advantage one Constitution saves that you make to maintain concentration on a spell when you take damage. You can perform the somatic components of spells even when you have weapons or a shield in one or both hands. When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, instead of making an opportunity attack. The spell must have a casting time of only 1 action and must target only that creature.
Shield Master: You gain the following benefits while wielding a shield: If you take the Attack action on your turn, you can use a bonus action to try to shove a creature within 5 feet of you using your shield. If you aren't incapacitated, you can add your shield's AC bonus to any Dexterity save made against a spell or other effect that affects only you. If you are subjected to an effect which allows you to make a Dexterity save for half damage, you can use your reaction to take no damage, interposing you shield between you and the effect.
Take the Pyromancy sorcerer subclass to bypass fire resistance and at later levels immunities.
Elemental Adept: When you gain this feat, choose one of the following damage types: acid, cold, fire, lightning, or thunder. Spells you cast ignore resistance to damage of the chosen type. In addition, when you roll damage for a spell you cast that deals damage of that type, you can treat any 1 on a damage die as a 2. You can select this feat multiple times. Each time you do so, you must choose a different damage type.
A tiny ball of bat guano and sulphur, a shield of any sort (If you got the Shield Master: feat) and if at all possible a Ring of Evasion: for automatic dexterity succeed.
Cast Delayed Blast Fireball, extend to the maximum (24 hours) by using 11 sorcery points (2048 minutes = 34 hours). Wait the 24 hours, or use spend your day regularly WITHOUT using Concentration spells, useful with the War Caster feat in case you get hit. The spell starts out at 12d6 damage, a round is roughly 6 seconds, the damage of the spell increases by 1d6 after every round, after some quick maths: 24*60 = 1440 (Amount of minutes), 1440*60 (amount of seconds) = 86400, 86400/6 = 14400 rounds. The spell after 24 hours has a damage of at the absolute lowest, 14412d6 of fire damage in a 20ft sphere, (With the Elemental Adept this is effectively doubled and bypasses resistance) change that fire damage to a different damage type to completely obliterate the Tarrasque or anything you deem not worthy of living, make sure to keep it a dexterity save for the big lizard. If they succeed the dexterity save, at minimum, 14412 damage / 2 is still 7206, enough to kill that big lizard 10 times over, if they crit fail, why bother calculating?. And if you are in the explosion radius, auto-succeed or succeed the dexterity check yourself and with the Shield Master: feat, you take no damage. Have fun camping right where the Tarrasque is due to awaken. Please tell me if I miscalculated.