easiest spot imho is on the third level. you don't even need to cloak.
after crossing the rope bridge and clearing out the camp at the other side, you pass an observation tower on the way out of the camp. head off in a vehicle along the winding road through the marshes and loads of enemies will spawn.
drive back to the camp and climb up the tower, where you will find a precision rifle dropped by a guy you killed earlier. look out over the winding road, use the scope and pick off the KPA one at a time (there should be at least six). they should still be far away and not really advancing so you don't need to use stealth mode.
even with the noise and their comrades dropping like flies the warning meter won't go up, and on the fifth kill your achievement will pop.
Retail 100%:  Halo 4 | Forza Horizon | Halo: Reach | Gears of War 1+2 | Portal 2 | Halo: CE | LA Noire | Crysis 1+2
Far Cry 3 | Deus Ex | Just Cause 2 | Assassin's Creed: I, II, III / Brotherhood / Revelations | Alan Wake | Fallout 3 | Prince of Persia
Rage | Dead Space | Mass Effect 1+2 | The Orange Box | LEGO: Batman 1 & 2 / Harry Potter / Indy / Star Wars TCS
XBLA 100%:  WARP | Limbo | Brain Challenge | Chessmaster | Portal: Still Alive | Sonic 2 | The Walking Dead | Wordament (iOS)