Rocket Wars

The shotgun weapon looks like three raindrops (not the one with 5 drops on it; that's the machine gun). You'll then need to get two kills with a single shot. Thankfully this achievement seems to be a bit buggy in a good way. I and others have had this achievement unlock after only getting a single kill and only hitting a second person with the shot. Either way, the strategy here is identical to the "TIMBER!" achievement. Refer to its solution to group the ships together, and then grab a shotgun and fire into the group. You'll definitely get at least one kill and will hit at least one other ship, which should unlock this achievement.