I believe the best map to get this on is 'Fire Storm'. It has a of storm drain at the lowest level of the map where raptors can be found.
There will be 2 raptors roaming around the trenches, which you will need to kill so that more can spawn. Once they are dead, you will need to find their spawn point (if you have not already). It is in the smaller, enclosed tunnel separating the two trenches - they spawn just inside a square cutout in the wall.
It will take a few minutes to kill all 20 because they do not spawn immediately. This means you will probably be throwing the game, so make sure you are playing a player match.
If you find yourself overwhelmed, you could either try asking your teammate to help you or try using a different weapon. I prefer using the knife, because once you have pressed RT, your character will start the short knife animation/cutscene, during which you will not take damage from other dinos.