Originally Posted by
Answerer
If he's not merely using DM Fiat, fooling Divinations can be tricky. But certainly not impossible and such a foe is very appropriate for Divination-favoring parties.
Still, anti-dragon measures will provide decent responses to the Tarrasque: you'll have the entire party flying, for instance, and you will be attempting to target low ability scores (traditionally Dex for dragons, but the Tarrasque's 16 isn't exactly stratospheric). The main problem is that the Tarrasque is immune to ability damage, which is one of the best ways of attacking dragons, and that dragons will force you to prepare for spellcasting that the Tarrasque lacks.
But ultimately, even prepared incorrectly, you'll still have good answers (and, if all else fails, this is a tactic that buys the BBEG just over 9 hours on the outside), and frankly the Tarrasque is a lot easier than any dragon of similar CR would be. The inverse (convincing them that they'll be fighting the Tarrasque and leave them with dragons) would be a lot worse.