If you are playing for the
ending, then you will probably not get this achievement.
However since you need to play trough the game at least twice to obtain both the
I would advise you to kill everyone on the playtrough that you are going for the bad ending
There are more than enough enemies to obtain this achievement if you just shoot anything that is hostile towards you.
However if you just want this achievement quickly you can load up the FRONTLINE
level and at the start of the level shoot or throw a grenade at the train cart with about six soldiers on it. After that clean up a few other enemies and reload checkpoint. Reloading checkpoint does not reset your progress towards the achievement!
Other achievements that can be farmed at the same location: