Perseus has a lot of health, and the way to beat him is similar to Pegasus: you must do as much damage as possible to him once he teleports to the island you are on, and if successful, he will kneel and turn orange, at which point you must use your special weapon to deal massive amounts of damage to him.

For more information on how to prepare and defeat this final boss, you can view our detailed guide for the Ancient Evil main quest, Greek Tragedy.