the giant robot thing

which giant robot? the sentinel boss fight? or the slightly smaller prototype versions of which i believe u have to fight 2 of em, also, be sure its telling u to press circle, and not square....

That is by far the hardest point in the game. I have beaten the game on every level, mastered all moves, gotten all trophies, and I still cannot make it through that in less than 5 tries.

It's extremely hard, a lot harder than it should be. I have heard a LOT of people have problems with it. Even quit the game because of it.

All I can say is hit it as fast as you can, never pausing, hit it consistently and fast. Even the slightest pause will cause him to drop it a bit. If your fingers get tired, take a break and try again. It's no easier on the easy mode. Hold the controller in any way that you can hit the O key as fast and consistently as you can.