The Cyclops is the first boss you'll face. He's a large, slow-moving enemy that throws bones. His weak point is his head. The Dagger sub-weapon can be effective here, as you can continuously throw them while dodging his bone attacks. Alternatively, use your whip by jumping and hitting his head.
Strategy: The Cyclops moves slowly from left to right. He will stop, wind up, and throw a bone in an arc. The key is to anticipate his throws. Stand at a medium distance, jump, whip his head, and then quickly move to avoid the incoming bone. If you have the Dagger, stand at a safe distance and spam it while dodging. The Holy Water can also be useful if you can land it on his head, as it deals continuous damage. Be patient and don't get greedy with your attacks.