Once you've got Big T down, you have all the time in the world: Ability drain doesn't heal on its own, not even for the Tarrasque. But you can't just miracle up some souls, because producing items isn't on Miracle's freebie list, either.

There are ways around this, of course: You could use Miracle to emulate Planar Binding to call an efreet and use one of its wishes to finish off the Tarrasque, for instance. But once you start down that road, it's a slippery slope to Pun-Pun, or something not too far removed from him.