Tarrasques are unintelligent ravaging brutes who have a lot of might. In case you don't know much about the story behind BG2 Watcher's Keep, Demogorgons are one of the most powerful creatures within the abyss:
a demon prince. Combatwise, such creatures not only excell at hacking and slashing, but they are also topped with magic and anti-magic capabilities. Also, these creatures are intelligent and don't necessarily destroy everything (if they can use charming/compulsion/diplomacy, they will use it before resorting to combat).
Discharger, you should really find either:
a.) a 3rd edition source for Demogorgons
b.) OR a 2nd edition source for tarrasques
Now compare your results with the appropriate edition, and you will find out that Tarrasques are only powerful for a weak/medicore adventuring party (so non-ToB party).
Please do it for us, it will save everyone a lot of headaches ~ including you.
-Galactygon