This is the first boss of the final chapter. Fire attacks are effective against his minions. The boss himself can be dealt with using electricity attacks. He's able to paralyze your warriors. He can also heal himself and allies. Finn i Tristis are the best for this fight. Finn has all the attacks needed, and Tristis is able to make shields and cancel turn cooldowns of the enemies.
Tristis' turns can also be used to consume potions. You can heal your partner, increase your damage or replenish mana, which is incredibly important in this fight - Finn will be constantly depleting it. Each of his attacks should either be fire or electricity. You should attack the whole team - some additional damage to the minions isn't a bad idea. It will also let you break their attacks.