This achievement is called (Very) Dead Tenant its for finding the dead rebal leader in chapter 2
this is in the next part after the turret courtyard section you will be fighting threw the apartments and yu will make your way to the attack where you have to free a combine solder from jail go down the hall behind him get a grenade and use it to blow up the machine in the armory making a hole in the wall. Go threw the hole you just made a look a the skeleton laying on the ground in back of the room to unlock this.