There are three stages to this fight. In the first, Orsino is weak and highly susceptible to spells with incapacitating side effects (such as Winter’s Grasp and Petrify).
Once you pass a certain damage threshold, the “head” of the abomination will leave the body and attack as a separate entity. You will also face a certain number of undead opponents. Try to gather these together and lay down AoE attacks to kill them swiftly, but don’t lose sight of your main target.
In the final stage, Orsino is restored to his full form – albeit briefly. One of his first actions is to rip off his own arm, then use it as a crude but devastating cudgel. He also regularly employs a Blood Drain attack, which freezes his victims in place as the creature feasts on their essence. Your opponent also has greater resistance to spells that impede his mobility during this concluding part of the battle, though that doesn’t mean that you shouldn’t still employ them for the brief respite they could offer. Preventative maintenance is vital in this final slog. Try to heal early and ensure that party members’ mana and stamina gauges are topped up with tinctures before they are exhausted.