So, for Hydra, there are two "must-have" priorities - block buffs, and provoke. The former for Mischief, the latter for Decay.
For the block buffs I'd probably go with Fenax.
For the provoke, unfortunately your roster is pretty thin. Husk in a Provoke set does okay, but he won't be consistent enough to keep it active. Giscard has one too, but it's also not guaranteed, and it's on a cooldown. If you can get both of those two into a Provoke set you might be okay.
For the remaining three spots I'd probably go with Trunda, Teo, and Apoth, since you have them at 60 already - but the team is far from ideal.
So - tl;dr level Fenax and Husk.