OK, this is how I did.
-Go to C Block.
-Turn off friendly fire. (Don't want to go negative 934 do you?)
-Get 3 friends
It'd be best to have 4 people, as there is more enemies coming your way. Grab the scatter gun and step back a few steps. Wait until they start coming and have your friends watching your back as the aliens come up the side vents. Just fire away and you should have 20 kills in no time, maybe 40-50 seconds it should pop. Seriously, there is 10x more enemies with 4 people.