This is easiest to obtain in a Gardens & Graveyards mission due to the length of the matches and the overall distraction of the Plant team as they try and defend their bases. You will need a level 3 or higher Engineer to unlock the Jackhammer ability. Tap RB to use the Jackhammer and you will begin riding it, then just make sure there are Plants close by and let them have it with your gun. Good places to do this are around key Plant locations that your teammates are trying to take over and that the Plants are trying to defend. Circle around behind them and shoot some unsuspecting Plants.