How do u beat "Nightmare"???

When he modifys the gravity, use your charged beam.
After that he'll fly aroung. He will eventually stop and move slowly towards you. Try to make him level with you while your on the latter thing. This way you can fire off the maximum amount of missiles. Keep dodging his giant body, and then keep trying to shoot him with at least 15 missiles while your on the latter.

then find the red security room, silly

are you stuck in an underwater area... if so, that is what you'll need to do. also remember to try to go places that looks like you cannot go to, you'll find it