I just defeated Krauser on pro mode. It wasn't as bad as I thought it would be. I died once during his second form because I got a little too greedy with my knife attempts. But it was weird though, once I slashed him to get him to kneel, I would just repeatedly knife him and once he was about to stand up, he'd fall to his knee again and I could continue slashing. He kneeled around 3 times before he finally went down but not once could he even move from his position.
I realized that the knife also works well for his first form. Walk in, then immediately walk backwards as he's about to slash. Once he misses, quickly run up and knife him. Do that twice and he'll flee.