About that AOE...I think that Lateral has an idea here.

It could be like the dreaded Long Gui from FF XIII, where the tarrasque just lifts a foot and slams it into the ground, and everything within range takes damage, gets knocked prone, and maybe gets stunned?

... ooooor...if we wanted to be really evil, you could give the tarrasque's foot stomp a great thunderclap ability (Evocation, Sonic, Spell Compendium). 3 save-or-sucks, targeting 3 different saves, with a range of...oh, let's say, everything within 300 feet that's touching the ground when he stomps. Save DC based on Strength, usable every 1d4 rounds.