Okay, here goes
Here's my attempt for someone that can take on the tarrasque in melee. Eventually, the Tarrasque would probably win on critical hits, but this person would at least stand a decent shot of doing it alone.
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Fighter 6/Wizard 4/Duelist 10
18 Str
18 Dex + 6 Gloves of Dex + 4 Manual of Quickness in Action + 2 Level = 30 Dex
18 Int + 6 Headband of Int. + 4 Tome of Clear Thought + 2 Level = 30 Int
AC: 10 + 10 (Dex) + 10 (Int) + 5 (Natural Armor) + 8 (Bracers of Armor) + 3 (Fight Defensive with 5 ranks of Tumble) + 10 (Elaborate Parry) + 5 (Ring of Protection) + 2 (Off-Hand Parry Feat) + 1 (Dodge Feat) + 1 (Luckstone) + 7 (Shield Spell) + 5 (Expertise Feat) = 77 AC
Add in Blur for an extra 20% miss chance on the shots that DO get through.
Between Expertise taking 5 off the BAB and Fighting Defensively taking off another 4, that gives a -9 penalty to attack rolls.
BAB: 18 (BAB) + 10 (Dex, Weapon Finesse: Rapier) + 5 (weapon enhancement) - 9 (Attack Penalties):
+24/+19/+14/+9 Attack, which means the Duelist misses only on a "1" roll, due to the Brilliant Energy Rapier. Critical Threat Range is from 15-20.
Average Damage: 4 Hits at 1d6 (weapon) + 3d6 (Precise Strike) + 5 (weapon) + 4 (STR) + 2 (Weapon Specialization) = 92 damage per round, average -- which is 52 per round after regeneration.
Equipment:
Amulet of Natural Armor (+5): 50,000gp
Bracers of Armor (+8): 64,000gp
Ring of Protection (+5): 50,000gp
Gloves of Dexterity (+6): 36,000gp
Headband of Intellect (+6): 36,000gp
+5 Brilliant Energy Rapier: 162,320gp
Ring of Three Wishes: 97,950gp
Manual of Quickness in Action (+4): 110,000gp
Tome of Clear Thought (+4): 110,000gp
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TOTAL: 716,270gp
Feats:
Ambidexterity
Two-Weapon Fighting (For Off-Hand Parry)
Dodge
Mobility
Expertise
Weapon Focus: Rapier
Weapon Finesse: Rapier
Weapon Specialization: Rapier
Improved Critical: Rapier
Improved Initiative
Off-Hand Parry
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Someone told me there may be errata for the Off-Hand Parry feat; I was unable to find any. If there is errata for it, I may need to alter something in this character setup.
EDIT: Screwed up the average damage.