I thought that the second time you fight B.O.X from Metroid Fusion was insanely hard. You have to climb across the ceiling while it jumps around and shoots seeking missiles at you, and if you get hit once, you fall into the electrified water below. To hit it, you have to wait until it stops jumping and slowly climb until you're right above it, then shoot it. If you don't make it there in time, it shoots it's missiles at you, and if you don't shoot those down quick enough, they knock you into the water, where you'll probably get hit by B.O.X while trying to get back up. I tried at least ten times before it finally beat it.
I never hear anyone talk about it, so maybe it was just me that had trouble with it.