-Block off the entrances with fake walls and barrels. That way, the demons will blow up the barrels but it will still affect the zombies.
-Use your grenades or Uzi for demons and large crowds of zombies.
-Use boxy rooms or littlel rooms because the zombies will move much slower trying to get through them.
-Make sure the devil is behind a zombie, then get close to the zombie but not directly next to it. The devil will shoot fireballs at the zombie, not you. You have to repeat this to keep the devil killing. It will almost kill the zombie, unless the devil hit it four times.
-Two people and the rocket launcher or railgun (rapid fire and long shot rail gun) are required for this. Turn off the devils, set object collision off, and put it on expert. Then, have both players stand next to the doors they come out of in boxy. Keep shooting at the doors when they come through.
Go to "Options", then select Nightmare mode and no devils. Use the boxy level, then block one entrance with fake walls and the other with barrels (after you get these weapons). After that, choose the rocket launcher and blow up the barrels. Keep shooting at zombies whenever they are created at the entrance. Repeat this tactic until you get a x999 multiplier.