The Gatling Gun is a bit more of a pain than the other two weapons. The Subway stage in which Nina is a boss is fairly good as you get one to drop in and have a chance at the enemies dropping another, however you have to be careful not to shoot them off of the ledges with it.
Another good stage is the nightmare train. Get what kills you can with the Gatling Gun at the end of the train and stand near the ledge allowing the enemy to kick you out of the ring. Personally I think this method is a bit slower, but it works.
Also, if you change targets with Rb and line them up so that you hit multiple enemies with a burst it will save you some time as you'll get more K.O's with less effort as well as saving yourself some pick ups.