What game are you playing?

[wankerness]

Yeah, I did practice with him a few times and had it down, but it's not quite the same practicing against him vs. actual enemies.

Every time you hit the parry button, hold it down. That way if you mess up the timing slightly you still block. The alternative is mash it as fast as possible, which works on a first playthrough, but it won't give you "perfect parries."