If you’re a Bio, gain the following skills: Leading Strike, Reform, and Crystallize. Pump all points into these 2 skills. Since you’re a pacifist, you need to keep your allies alive and buffed. Simply use Crystallize and keep the main Hydra and an arm frozen, using Leading Strike on a Crystallized enemy to gain Focus. Your allies will attack the enemy that isn’t Crystallized, and in between Crystallize waves, use Reform your allies if they need to be Reformed.
Your allies can kill the Hydras and you don’t have to attack them once. Be sure you equip Roald with the Ember Rifle as well.