Easy Silent But Deadly Achievement for Halo 2 PC
This is an easy way to glitch and get the Silent But Deadly Achievement for Halo 2, PC edition. To make it even easier, grab the Halo 2 Trainer from www.cheathappens.com.
First, load up the Quarantine Zone level. On your way to the room shown in this video (its basically a gigantic moving platform) do your best to grab a rocket launcher or some other long-range weapon. If you did decide to use the halo 2 trainer, turn on your super shield and your super ammo reserves. This is just for the purpose of getting to our next objective quickly and easily, and has no bearing on actually getting the achievement.
Now for the bit that makes this the "Easy" method:
Once you find yourself on the moving platform, go to the rear and grenade jump up to the platform. Run up the ramp and you will eventually fall though into a section of wall in the middle of the platform. From here, just rain down rockets on the Flood below. They will be unable to see you!