The Tarrassque is immune to most of Xykon's go-to spells. If it came down to a Xykon vs T fight, though, I'd expect Xykon to whip out some ability-drain spell (maybe epic, or maybe just a mid-level spell we haven't seen him use yet) and drain the Tarrasque into a non-threatening state (the Tarrasque is not immune to ability drain for some reason, despite being immune to ability damage and energy drain.) The main reason I'd expect Xykon to trounce the Tarrasque is because Xykon is an important character in the story, and the Tarrasque isn't.
Which, IMO, means it's more likely the Tarrasque won't show up at all. Or if it does show up, it won't be the "final" defense of the gate.