Halo 2 Cheats

Find any level with a grunt or two sleeping, or even an enemy around that can be easily assassinated from behind without alerting anyone around (This enemy should preferably be right after a checkpoint). After you've done this, assassinate them. Be sure to hit the enemy with the reticule not directly on the enemy, as there is no chance for the enemy to fire a few shots off while dying and alerting others nearby. Once you've successfully killed the enemy/enemies, reload to the last checkpoint. From there, just find those enemies again and kill them as before. Keep doing this until you get the achievement, but without getting caught in between loads.

It seems that the game has a glitch, which doesn't reset the "kills without being caught" meter at every reload. May as well exploit it.