Get the green armors as soon as possible. If you have all of the amulets, don't go to the sphere unless you have collected at least four as lots of monster remain and you'll need the amulets to battle the end of level guardian. He's a bit soft in the head, so time your jumps carefully, and hit him there.

There are four planets to visit. Go to everyone if you want to get maximum points. Again, the green armor is very useful, so get it as quickly as possible. There's loads of ammo for the guardian of this level. Unfortunately that means there's loads of monsters to deal with. The back of the guardians head is it's weak spot.