To unlock this achievement quick (max 30 min), you have to find a boost guy, then go to the multiplayer level Kowloon Teahouse. The player who wants the achievement have to go upstairs, then he have to swing with a chandelier to the others side, there will be an ledge on which you can walk for searching the big machine gun. Wait until your gauge is full, be side by side, then aim his head, press RB, shot on him (he will quick die), and quick press RB, you will lose less than the half of your gauge. With the time of the others players appear your gauge will be full again.
Sometimes the big machine gun will not appear at start of the match, just kill the others guys 3 - 4 time and she will be there.