Ok Here's a complete guide to netting this tricky achievement.
Step One:
Play through all three campaigns making sure to shoot every enemy you come across. Make sure you shoot the enemies in the Football levels as well as the Destroy the Object maps.
Step Two:
Credit goes to Celtic Force for figuring this out. Thanks Celt.
You should still be missing Harry, Tom, Niels, David, and Joost, the developers who only appear during one stage as mini-bosses which do not count towards the achievement.
1: Open the Windows 8 Charm Bar. You do this by moving your mouse cursor to the top right or bottom right of the screen and the Charm Bar will slide in from the right.
2: Select settings then go and select credits.
3: These characters will be waiting there for you to kill, they won't even shoot back.
Step Three:
If you are still missing any enemies at this point you most likely missed one on a Football or Destroy the Object level. Replay the levels and focus on shooting enemies rather than the objectives.
Step Four: At this point if any characters are missing you should be able to determine what level they are on from the enemies around them. Just replay the levels with the characters next to them and you should find them. You could also play the MP Challenges or Survival Mode as enemies spawn randomly.
I know there is another solution up but it doesn't address the entire achievement. It only addresses the most common reason for missing the achievement. Some people have yet to locate the other enemies as well.
Edit: A lot of people are missing characters that spawn in the level "Small Rooms" There are a lot of different enemy types that spawn there and as far as I can tell they will not all spawn in one playthrough of the map. There are a few that spawn infrequently on that map so you might need a few playthroughs just to get them to spawn.