The second boss of chapter ten is Nox. It's your former friend that betrayed you. She turned into a sea monster - serves her right if you ask me. Her tentacles and head are three separate "enemies". You can attack each of these individually. Nox can create shields and prevent you from slowing her turns.
Finn is the best idea for you can attack this boss with lightning, which are pretty effective. Chose Robert as your wingman. He won't deal massive damage, but he's able to cancel her turns. He's also able to slow her cooldowns. This will make Finn's job easier - his attacks won't be broken.