This achivement is surprisingly easy to get, the hardest bit is enabling the DOA power-up. Following the three steps below will activate the power-up.

1. Enter free play mode in single player and imidiately "right click & press F5 simultaneously". This will bring up the console for the game.
   
2. Type doa into the console and press enter, you should see the command "DOA enabled" in the console.
   
3. Then "Right Click & press F5 simultaneously", and get a shooting.

After this point you will want to get all the perks relating to the "Ghost" power-up. These being:
Apparition - Doubles the chance of a ghost power-up appearing.
Fire Team - Combines with the ghost pickup to equip them with shotguns.
Strike Team - Combines with the ghost pickup to equip them with AA-12 shotguns with armor-piercing shells (Requires Fire Team)
Mirror Image - Combines with the ghost pickup to create two ghost fighters.
Wild Bunch - Creates four ghosts with the ghost powerup. (Requires Mirror Image)

Alongside these power-ups the following two also help alot with this achivement:
Luck - Dead zombies will be more likely to drop powerups.
PowerUp Extender - Extends the time for any powerup by 25%.


Once this is done you should find it very easy to get the required 50 kills for the achivement.
Good Luck!