Don't use any melee weapons against this enemy. The best way to defeat this boss is to use flame spells. Any spell will do as long as it is a fire type. You'll easily notice its effect because the ape will continue to burn after the initial attack.
The ape's attack is also very difficult to dodge. Don't spend time trying to avoid it; instead, fight him head-on. He will be stunned by fire spells, allowing you to execute another attack before he can recover. This way, you can avoid being hit plenty of times.
Submitted by Carlo Hipolito