Prepare to have a rage stroke on this one, because this is some absurd bullet hell nonsense.
No matter what, you can expect to do this heroic fight a couple of dozen times just to learn the patterns alone. Position yourself slightly away from the energy ghost that lands near you so you can avoid it when it splits in two. Meanwhile, hit the boss with arrows from the ground.
When the fishy, Cthulhu monster is summoned, learning his wonky hit box is critical so you can deal damage to him quickly without having to move far away. When he summons his magic shield, you can often stand directly inside him without taking damage if you move to the side.
The red arcane bullets filling the screen will make this battle incredibly tough, but there's a glitch here to take advantage of. They only seem to damage you if they hit your lower body. You can avoid damage entirely by walking at a measured pace and letting them pass through your head! Expect that to get patched at some point, however.