this is called "sweep"
you need to play a gang war and win all 5 team gang war parts.
It's not too bad, I got this on my first try, just be on a good team. Its 8v8 so there isnt much you can do by yourself except play the objective or cover someone who is playing it.
EDITS
If you join in halfway and your team "sweeped" the other team, you don't get the achievement. Thanks to RSDAY for that.
and to all others who are mad about not being able to get it, I'm sorry I can't write a guide for unexpected online play. It pretty much is luck, I know I could write how to win each gametype, but they are pretty simple to understand.
Jameson says "I can confirm that this doesn't work in at least one case. I joined in the middle of the first round, but the game was the one where you only get one life, and if you die you're out. When I came in, I didn't get a life. I was basically in spectator mode for a minute or so, and the round ended. We swept all rounds, no achievement. Perhaps if it was a different game type and I could have participated in the first round, it would have been different."
Edit**
bblcreator8790 "You can start the match with as few as four people (2v2). It turns out that now this achievement is actually easy to boost in Free Aim because virtually no one plays the Free Aim playlists these days. I set up a boosting session a couple of days ago and we boosted this achievement for 5 people with no problem - didn't get a single random player in either of the matches. We were also able to boost other achievements while going for this one."