This one is simple, as you are playing the level 'The only easy day... was yesterday', you will get to a part where your objective is to 'secure the derrick building on the top deck', you are then told to use the thermal guns which can be picked up at the top of some stairs just before a fair sized battle, then kill 6 enemies in a row with said gun and achievement is yours.
If you somehow find this hard, after picking up the thermal gun, go back down the stairs and wait for bad guys to come to you and simply pick them off.