This solution works for every achievement.
Play the retro mode on the easiest setting and complete all 50 levels. This isn't a tough ask, and if you die you'll restart with 5 lives on the last level you played.
Once you have finished the game select co-op mode, connect a second controller and place it next to you.
Select which retro game you need to complete and load into level 50 by pressing continue.
My advice for this level is to hide behind the first block allowing the computer to destroy each other. You may have to peak out and hide again to get them to shoot.
If or when player 1 is destroyed simply pick up the other controller and wipe out the remaining foes, for some reason they don't shoot at the second player. As long as the second player hasn't been killed by accident you should mop up the game in a matter of minutes.
With thanks to BiLLzuMaNaTi for this wonderful video walkthrough. He perfectly demonstrates the above solution.