It is recommended that you do this achievement with at least 3 players, but 4 are definitely better. For the achievement, you must complete round 20.
I recommend doing this on Octane, and camping in the Strip Club, as most enemies will come through the front entrance, and you can keep one person in the back just in case. In between each round, make sure you pick up whatever weapon drop that you are comfortable with, but more importantly after every 5 rounds there will be killstreak drops, which show as blue crates. Make sure all members of the team run around the map and grab as many as possible each. Also, make sure that you use them when needed, and space them out across the next 5 rounds until you can get more. It is best if you place the turret in the main doorway, and call in choppers and missile strikes near the beginning of the round. Keep communicating with your team when necessary, and you shouldn't have too many problems passing round 20. Also, make sure to revive each other if you are downed.
Another tactic involves playing on the map Stonehaven, and camping inside the small building next to the burning house as there is only one entrance, and the enemies will all line up easily. Watch out for riot shield enemies though.
In total, the achievement should take around 20-30 minutes.