its hard. but if you find the order to beaet the bosses its easy. like you beat tornado man at first (for example) and get his weapon then you go to the level where the boss is weak against tornado's weapon. if you have the right weapon the bosses are really cake. the levels themself are something else tho as they are really hard or hard. ive beat it up to dr wiley's level and that shit just got hard and i lost interest in it. good game but hard.
you can find the boss/level order on gamefaqs or ign guides.