Swarmkeeper
Hero
The tarasque cant be targeted by spells
Not exactly. Here's an excerpt from the 5e MM description...
Magic Resistance. The tarrasque has advantage on saving throws against spells and other magical effects.
Reflective Carapace. Any time the tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the tarrasque is unaffected. On a 6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.