Well the Tarrasque basically sucks, that's an established fact, essentially.

What makes Big T dangerous is honestly you should never just run into him or find his lair, no no no...

Ideally you should never even hear of the Tarrasque except in legend..... and in rumor that the BBEG is going to wake him up and kill everyone.

Cut to epic battle vs. BBEG:

BBEG finishes ritual *just* as PC's (barely) defeat the villain's minions. Big T goes buck wild and the PCs get to fight him on the DM/BBEG's terms, in a place of their choosing, with depleted resources.

Then Big T is well worth his CR.