Floating shield trick:

Throw the shield in front of you, then restart the game with Option 1 and Pause. When you restart the game, your shield will be floating in front of you. As soon as you fire, your shield returns to normal. (programming glitch?)

Power-ups on final level:

Even though they're not displayed, you still have them, unless you die.

To kill the final monster:

Jump up and shoot him in the face. When hit he will flicker. Takes about 20 shots.