You don't need 3 friends for this, just 4 controllers, even Guitar Hero/Rock Band instruments will do. Basically anything with a start button.
Start a level normally with only one hero on your team (use the team set-up screen to remove the others), then press start on the other controllers and select Co-op. The game will warn you that there are no heros available, but when you exit the level, the achieve will unlock.
This works for all the other multiplayer and arcade achieves too. Just make sure you kill at least 3 bad guys before exiting the level on arcade modes.