luckily kills in custom escape count for the achievement then, if you are not interested in exp and cards but only in this achievement, this is the fastest to grind it.
I made a map in in few hours, it works well but I'm sure it can be improved, if you have any good suggestion share it in comments and I will update it
The map name is "Rest in Pieces" and its ID: 7S17K5 , this map is build for keegan and his cards for explosives damage, the only weapon you will find on the map are shocks granade and salvo, you have to fight 10 rooms each one with 100 juvies, if you follow my strategies you will earn about 1000 kills in 11 minutes, don't worry about venom it's disable, no skill required only a lot of patience
Juvies will be always in the on left room, they will not attack you istantly because they are set on defensive, the best way to approch every room is with a full salvo and 4 shocks granade, trhown a shock on the floor of juvies room and switch to salvo, shoot 3 rounds at upper level of juvies room and keep doing that until you kill all them 100.
On the floor you will find 2 more salvo and 4 more shock granades to be ready for next stack of juvies.
I made a video because my english is not very good and will be easier to understand:https://gamerdvr.com/gamer/zyx/video/107873393
When you clear last stack of juvies, don't finish the escape map but restart it, kills counts instantly and you will save some time avoiding loading times.
Edit 1: Blackout 4171 point out that an high level Cole with the gold card equipped is a good alternative to Keegan (in this case check also FuzzyRabbidDogg's guide maybe aggressive juvies are better for your playstyle)
Edit2: as suggested by RedHeadMexican4 with a good JD you can have at same time bleeding damage and more ammo for your salvo
Enjoy the grind.
PS: sorry for my english but it's not my native language