Level nine just gives me a few more defenses (i. e. ways to stay the hell away from it) and tries. My build revolves around Summon Undead IV and V. I summon allips.
The Tarrasque is immune to ability damage, but not ability drain. I sic the allips on him. 14 points of wisdom drain later (six hits on average), and he's down.
Granted, he'll get attacks of opportunity on the allips. But since they're incorporeal, he'll only be hitting half the time (and wouldn't hit even that often if his natural weapons didn't count as magical).
EDIT: Oh, and the good news? He's still alive and regenerating. Nothing kills him. You canharvestmine tarrasque steaks for the rest of your days.