Hot Shot - Gain promotion to the rank of Gunner.
The two solutions here provide very good information, but do not help people who are trying to get this achievement since the game's community is basically dead.
So, a boosting method to get this achievement is to first, get 3 other partners/xbox's, as this achievement requires 4 people minimum.
This achievement is only obtainable in ranked matches, so you'll have to deal with the waiting in the lobby for the match to start.
Anyway, you'll be boosting in "Post Grab Battle" and the map setting should be "Small."
Once in the lobby, get your other three xbox's/boosting partners in.NOTE: If you are the host of the game, press or in the lobby, it should bring up a little box, this blocks other people from joining, which will ensure an easy boosting match without the worries of pesky randoms.
Get everyone else to vote for the map "Cube", as this is the smallest map in the game.
Once in the match, whoever is going for the achievement must capture all the data posts as fast as possible, you will obtain around 4300-4700 Points, which should allow you to obtain the Gunner rank in a matter of a few hours (It usually takes 8-10 hours of constant boosting)
Your partners can go for kills if they need this:
Just follow this method and you'll have gunner in no time.