Blow up 20 fire extinguishers. (Single Player)
These can be found on walls around the different levels, there is way more than 20 throughout the game. A quick way to get this is load up mission 7 "KATN" and work your way through the level. You should reach a room about half way through with three soldiers in it, one of which is bending down fixing a box on the wall.

There are five fire extinguishers located in this room, proceed to shoot them all then reload the checkpoint and you should start just before the room.