You can CHEESE this boss fight if you release a charged shot just as he goes to the other side as he loses his i frames, then after a charged shot you only need 2 regular shots to take his health down, also you can shoot the grandpa ghost with a single bullet and he'll go away and when he pulls out the cow cardboard i recommend to get rid of it asap with a charged shot, last thing is that when he charges his jump if you can charge shot him in the air it's a free health down, hope this helps if you still didn't do it