How do you defeat the troll in Castlevania-Lord of Shadows?

I just rented the game for PS3 and I can't beat the Troll in the 3rd quest. I actually easily get him to start glowing when his power bar is almost gone but then when I do the "killing blow" it never works. I did what the instructions say at the top of the screen "Get close and press R2 button" and the character jumps on the troll and puts his weapon in the trolls head. There are white circles all around the character and troll but then then the troll just throws me off and then does a elbow power drive and I'm dead. I've tried hitting R2 to repeatedly, holding R2, hitting R2 and then hitting the O button, the triangle button, the square button the X button....Everything! So how do you defeat the troll??????