I've got a theory and made a new Poll post, be interesting to see what the results show, but you're the second person NOT to get the achievement who got one or more of the Slimer in a Campaign...
Need help with: Driver: San Francisco | Halo: CEA | Halo 4 | Resident Evil 5, Campaign DLC
XBO - Halo: MCC |
When sending a friend request, send a message and please mention xboxachievements.com, the game and the reason for adding me. If I've completed it, I've probably uninstalled it, sorry, all my advice is on these forums.