The Mulcher is a heavy turret-type gun. For these kills, you will need to have it equipped, take cover by pressing A near a low wall, hold LT to mount the gun and RT to shoot it at your second controller player. Simply shooting with RT does not count as you are carrying it, not mounting it.
For this and any other weapon achievement, if you don't get all your kills in the campaign, the easiest way to get the rest is to setup a Local Multiplayer match with a second controller (or nine bots) and change all the guns to whichever one you need. This will be referred to as the "Organ Grinder method of boosting" in many following achievements.