I have ninja gaiden black, absolutely amazing game. The first boss is a bit tricky but keep using wall running attacks so that you end up away from him, and you should make easy work of it. Also remember to roll and jump!!! Key in most boss battles.
I find wall running again Murai to be fairly useless. I generally just stay close and roll away from all of his attacks and quickly hit him a few times, before rolling away again. Rinse and repeat until done. It can be a bit slower as it's not so aggressive, but with good reactions you won't get hit at all.