Well, definitely could improve.
But, you did a good job. Good sound syncing, and some good bits, for example the ninja bomb throwing, was done quite well.
However, too many flaws.
There should be a stopallsounds(); script at the end of the movie, incase people want to replay it without the song screwing up.
Most of the animation was choppy. When someone falls down, they aren't standing up one second, and down the next. Take into account poses and anatomy, i.e. the back would be bent, arms out.
The backgrounds need some details. They're there for the whole thing, they should look at least a little pretty! this can be helped with gradient backgrounds, and a little bit of shading.
But I liked the ending, last frame was good, and title was good.
I realize your still learning, and this is a good start. My first few animations were atrocious, I'll never let them see the light of day again.
Anyway, next time probably include a menu, customize your preloader, and make the animation fluid. Good try man!