This achievement is most easily unlocked with the following settings:
20 minute match
1 second respawn
You can see the other players' dots on the map
With such a confined space, with a bit of skill you should be able to get 20 kills in the needed time limit.
Boosting Note: This one is a lot easier to boost than the rest. When you get to a ranked lobby, if there is only one person in there, or if you happen to be the host and one person joins, ask them to do the achievement with you. Many people are willing to boost this because of it being so easy to do. If you do find someone interested, you will be able to see each other on the radar, so it will be very easy to find each other in the confined space of the prison. Personally, I found someone to boost this, it took us about 14 minutes 30 seconds for both of us to get the achievement, then we just played around for the last 5 minutes.