Huntdown

For the first form of the boss, I advise you to take an indestructible sword, which falls from the red ninja after the samurai with a machine gun. He has very good damage and the boss is simply made to kill with him. In the second form, you can make the first blow with it, but then it is better to take another weapon.
Video guide on how to beat the last level and boss without taking damage. In any case, it will help on maximum difficulty.