When cloud entered soldier he was made into a Sephiroth clone but was ruled as a failed experiment, thats why he was just put into the infantry. You do know that Sephiroth didn't kill Cloud the first time on purpose right? And yes he did have Holy on his side during the final battle in the game, thats why Cloud was waiting for White Materia to turn green. Angeal and Genesis were givin the same strength of Jenova cells, they were just not given as many.
If your talking about the battle from AC the entire point of AC was for Sephiroth to mess with Cloud.
But back to the actual post... Its already been stated that Sephiroth surpasses Mercer in almost every way.