You can set it in the config. By default they have 1k hp.
However, the trolls do not take damage like any other mob. They don't care what kind of weapon hits them and they currently ignore bows of all sorts. The ballista crossbow works somewhat (which is unintended) but the stationary ballistae work better. Tiers don't matter to it.

To perpare for the enwave batte u better buy lots of hp potions and a long wepon to hit the eye.