Spoilers for zombie island DLC (even though achievement kind of does that in the name lol)
This achievement is not possible to miss. It's gained through doing the main story of the zombie island dlc and is gained when
This shouldn't take more than a couple hours of play time to get and is pretty easy and quick if you ignore all the side quests up to this pont.
Killing
shouldn't be too hard but as always playing with friends will make this fight easier if youre finding it difficult.