Load up "Coagulation" in custom games on the Halo 2 Classic Maps and head over towards the overshield water area off to the side though more towards the middle of the map. It is marked by three trees, and by the red team base. It will be on left of the wall in the drop-off into the water. Check out the images below. Finally, shoot at it and wait a second until it pops.
Video showcasing this Achievement, starts at 3:32