ANKOKU
You probably won because the Charge beam did some (not a lot but some) damage each time you froze them.
Here's my tip for Nightmare on hard when you first encounter him. His laser spin move is a one hit kill. This is the best way I have found to avoiding it, it may not be the fastest, but it works. I move over to the spot that you wall jump to get to the top floor. (it's to the left of the exit on the top floor) When he does his laser spin (whatever the hell it's called). I jump down it. This way it gives me plenty of time to recharge my missiles and life (if needed). Keep shooting him while in between the wall jumps. Then when he tries doing the laser spin again, wall jump back up and move slightly away from the gap, (That way he can't hit you) either refill or charge for the super and wait for him to pop up. Note: This method mainly for when he has the gravity thing going. This also works for when he charges for you.