Pretty easy, really. All you need is Gate. And a way to keep out of the T's reach.
Start off by casting Gate. Use it to bring in your HD worth of celestial or fiendish stirges. They're still just 1 HD each.
So now you oughtta have at least 17 stirges ready to do your bidding. If you got another Gate, cast it. The more, the merrier.
Now, send them to attack the Tarrasque!
The monster is big enough they can all EASILY attack it.
So they latch on. Melee touch attacks. The Tarrasque's armor is all natural. It's touch AC is terrible.
It attacks. 1 hit, one kill. No cleave feat. 6 stirges die, die DIE!
The remaining ones...suck. The Tarrasque takes 1d4xstirges in Con damage.
I think you see where this is going now.
Con zero. T "dies." Not REALLY, but it hits -10 (assuming you consider this a "death attack," which is actually fairly generous since it's not specifically noted, and the rules are clear that ability damage is not subject to regeneration)
Now you just make a Wish.
Bam.