Make so you have to avoid him eating people. Also give him overrun. Throw in a puzzle that requires a few people to do, and make it so the rest can't just settle with a "you tank/kite him while the puzzle is done" so it doesn't end up a pike happy place like reaver. Also make it so he'll smash up terrain that can really hurt if you're in/next to it. I mean this is the Tarrasque after all, it needs to rampage!