Peishe can be challenging at first, but once you get the hang of how you have to move, he becomes rather easy.
His movepool consists of two attacks: He always starts with swimming to the height the character is at and then starts spewing a bubblebeam. The beam covers a large area in front of him, but he always leaves an opening at his backside when using this move. Use this to your advantage and smack him.
After a short while, he will stop shooting and will instead headbutt one of the walls, prefering the side the character stands on. The actual headbutt obviously deals damage, but it will also cause debris to fall, which is also able to damage you. Swim upwards to see how the debris falls and slip through the openings.
The Summon Stone can help immensely with this fight; the ghost can swim under Peishe and wreak havoc there, depleting his lifebar quite efficently.