Peter being damaged figures that his body might be composed of quite a bit of magic. So he tries to somehow fly over to the crystals and sink in a bit to repair a bit of his mass. If this doesnt work he tries to consume one of the herbs around the cave.

After this he remembers something. From the abilities the demon king gave him his eyes they must be one of his strongest parts of the body. He goes near the crystals using them as a mana generator for training. He first of all tries to fire a mana orb from his eyes. Then he tries to make the mana orb thinner. This while reducing its power would perhaps allow room for something better. He then tries to lenghten the mana orb in an attempt to create a mana ray.