This is actually a hard game. A good way of beating this game is by shooting enemies ahead of time. A few ninja stars thrown while you move keeps you alive. To beat this game hesitate about a half a second and then move and attack. If you move too fast you get hit way easy in the harder parts of the game.
I played this game without a save state and can make it to the last level. In some cases you have to memorize the patterns of the enemy. Most of the end bosses are beaten by lowered jump shots. There's a boss with revolving statues which is pretty hard. You gotta attack and jump with ninja stars which usually gets them to spin 3 or 4 times and then hit your ninja magic which if you press the buttons while the animation is going it will destroy the statues faster. You then get to jump and attack with the stars. This is on the set 5 version of the game.
The final boss is way diffcult. I don't see kids beating this in one night. First you gotta stand still and then move under his jumps. You then gotta throw stars at him low and try to hit his feet. You then gotta attack his whirlwind form immediatly with your fists and hop over his boomerang attacks and keep moving forward. He then forms shadows of himself. I use my ninja magic here. He finally changes form again and charges at you. Your only hope here is to push your attack button repeatedly evenly. The game then kicks in the ending.
Your best way to get extra men are the bonus stages which can be beaten and a high score. When you clear a stage you can even go back in the level and fight enemies again to get to the higher score faster. This is a harder path but gets you a extra man earlier. Each enemy in the harder levels require you to keep ducking, jumping quick or timing your shots just right. This is probably one of the hardest arcade cabinets around. Another hard arcade cabinet is Sega Ninja.
There's also a part where you can rack a few hundred points each time in the level where the guys in the wetsuits attack you with swords. At the end right before the helicopter lifts off you can keep shooting enemies as they fall down. This is good for another thirty seconds or so. I also posted a screenshot.