It takes time to master the game. You'll fail a lot, but with time you'll become a lot more proficient. You'll simply have to work towards that point. If you see a part of the level that is hard to reach or seems impossible, give it your best shot. The chances are you'll find something valuable like an extra life. You don't need to kill every enemy in the level to complete it. If you don't feel like going the extra mile, don't. Usually you can shoot most of your enemies from the end of a screen before they even think about reaching you. Use this strategy whenever possible. As it is with any boss, it's all about analyzing his movement and using the prediction to get him. Don't touch the shooting boxes, as they can cause you harm.