Its not too difficult. Its like any game - practice makes you a lot better. Trust me!
When I got the original Ninja Gaiden for the XBox, I got stuck at the first boss (on normal!) for a few months, but I kept trying & trying, and eventually I beat the entire game on Master Ninja. Now I can pretty much fly through the whole game on Normal without any challenge at all.
The same can be said for Ninja Gaiden II. Once you've completed it on Master Ninja, the lower difficulties become a breeze. You've just got to get into the right mindset to do it. If you say its "too difficult" to yourself, you've already lost.
Personally now I'm battling through Mission Mode on Ninja Gaiden II. It's a hell of a challenge but I
know I can beat it, and one day I will.
For me, this is the best game on the 360 by far.