I've used power bombs on OP before and they hurt him bad; the bomb just needs to be close to his weak point (core) as possible when you set it off, so that the explosion hits the core for a long time.
Best results were, roll up to the edge of the phazon pool he's in (without moving so close to the pool that Samus takes damage), place one regular bomb, and when it blasts Samus into the air, drop a power bomb at the peak of the jump. Morph out and switch to x-ray as usual, and then back away a little if you can manage it, cuz OP thrashes around like crazy when he gets hit by a pbomb and will prolly dog you if you don't)