Here's the easiest and quickest way:
Episode: Lunar Apocalypse
Level: Overlord (final level)
Start the level and activate noclip, all weapons and god mode if you like. Press
1. Go down the hole to the room filled with alien eggs and explosives (it's easy to find with noclip).
2. Choose the Devastator and save your game.
3. Turn 360° while shooting.
4. Load your game.
It should take around 10 minutes to kill 1.000 enemies. Each run takes around 10 seconds (with menu and loading) and gives you 15-20 kills.