Simply use your gun to kill an enemy behind cover. Use a sniper with FMJ, find thin cover (wood but your sniper will shoot through 90% of objects). Engage combat, swtich on your DNI then return to cover. This will light up enemies all over your weapons thermal sight whether you can see them or not. Continue shooting or swap to an alternative cover until you kill someone. If your DNI is not picking enemies up throw a flash or jump in the air.