Fight all enemies as they come on screen. Grab certain power-ups for better fire power and get some that give you a “spirit self” that will make you two or three times as deadly.
In addition to collecting power-ups for additional ”ninjas” you can power up your weapons with blue icons. For example, collect one for the ninja stars and instead of shooting them in a straight line, they go four ways. Collect one for the bombs and you’ll throw three or four at a time, causing massive damage.