(I forgot what case this is but I'll edit it if I find out, or someone can comment the case to me and I'll put it in)
Nearing the end of the game, when the Gas Zombies start coming, you'll be sent to the underground to save Rebecca from Sgt.Boykin.
The boss fight isn't too bad, just avoid his machine gun and the grenades. Getting close is key but try to avoid his button-mashing attack because he can take a good chunk of health off.
When you kill Sgt.Boykin you will get the achievement.