Great opening... poor finish....
So, you have this great premise, the ninja is going to fight this other ninja = good. You animate this pretty well with the ninja killing galls = good.
The sound you have is a little off sometimes: why is the sound of the sword louder than the narrator in the beginning? = not so good.
Your ending was far too short, if the evil guy had killed all the other ninjas they may as well have been cardboard boxes because the guy died in one slash = bad.
It just needs a more exciting finish, this ends too quickly.