I've had ONE. This thing is almost impossible to get seeing as you're only against a team of 6 and everyone tends to work solo, so it's as much down to the enemy being bad and clumped together as it is you being good.

Mine came from a ludicrously lucky Air Stomp on Skyline. Was trying to level it from the elevator shaft roof and saw two guys below me shooting off the lower roof. Went for a double kill and somehow got 5 instead! As there's no theatre mode, I have no idea what happened, but I assume a team of 3 must have ran in and joined their friends as that exact moment. Bizarre and stupidly fortunate.

Since then i've had two more chances where i've flanked a team doing a conga line, but been killed after getting 3 or 4 in a row, picking them off from the back one at a time, as all it takes is for one person to turn around.

Silly it's so hard, especially as there's an achievement tied to it ("The Cleaner")

Mine was on Skyline in the bigger greenhouse in the middle. Got a double C4 kill, was watching off to the side some. 2 people came in the door single file to check it out. Shotgunned them. Guy tried to melee me in the back but I somehow spun out of it and ended up behind him to get melee on him. Some skill, lot of luck, and bad timing for them to all be near each other.

I called everyone Muppets after getting MVP on crash site...

Oh wait your referring to the killstreak one...
Yeah good luck getting anything more than 4 kills in a round. Hell I can't even snipe majority of the time.