Ok, I just got this one, using a boosting method. Me and my buddies got Sandbox (you need 8 total people and you can sign in your guests - I had myself and my guest, my buddy had himself and three other guests, and then we had two other people just playing on their accounts). The best way to do this, is to:
First - Make sure you use the Missile Pod!! (It has 8 bullets and needs no reloading time.
Second - make sure everyone is close together, be sure to shoot whoever is actively playing first. (Also be sure to take your time after each kill, wait 2 seconds to conserve time for your 5th kill to get back)
Third - Hope that your aim is good and that you are able to hit the 5th guy, even though he might be far away.
Fourth - Hopefully you will see "25G - Killtacular" show up on your screen!! :D