Defeat First Boss Easily
When fighting the very first boss of the game, evade his attacks as much as possible. Walk on and around the vicinity near the end of the conveyor belt. He will accidentally step on the conveyor belt trying to follow you and attack you. When this happens, kick him as he is on the belt to make him fall off to his death.
Invisible Man
Make sure you have a bat on the second level. When you get to the area where there is a pile of beams and a chain link fence, walk up to the man standing close to the beams. When he starts walking toward you, climb up the chain link fence and walk to the end of the fence. Walk back and start swinging your bat at the end of the beams until you get seven hearts. You will hit the now invisible enemy, get all your hearts back and be more than ready for the next level.
Ignore Second Boss
When the boss appears at the end of the third level, climb down the ladders in this level. You will pass this level without having to fight the boss.
Machine Gun Boss
At the end of the final level, you will have to fight a boss with a machine gun. Dodge his machine gun attacks. When he stops attacking for a moment, grab him and throw him. Once he starts to stand up, kick him in the face. Repeat this process a few times and he will die.