First you must unlock a Nightmare Arcade by gaining at least 12 stars within the available levels. Nightmare is a bit tougher. Enemies take about double the shots to kill them. Also, there are no waves. They just keep coming without you having much time to prepare. Because of this, you should get a lot of points. This is suggested after you have unlocked all Nightmare levels and know which one you are best at. You will need to know where everything spawns, and stick close to an ammo drop. So make your rounds early to pick up flares, guns, and flashbangs. Then camp around at an ammo pickup.