This achievement is most easily done on 13TIRALL.lvl
. You may
have to do this from level select and not the first time playing through.
From the start of the level, duck into the alley, grab the throwable and disarm the rifle enemy. Beat him to death, and pick up his gun. Turn around, aim down the alley, and try to align the enemies to get the burst to kill all 3 of them. This may take a few tries, and you must make sure one of the enemies doesn't shoot the other!