think by far the quickest and fun way to get this is Juvie madness mode. As heavy, soldier or a scout..
Aprox 30 minutes to do 25 waves, should easily get 250 kills so 2-3 juvie madness's should easily net most ppl this, even with randoms.
Stick a 200kill bounty on for 25k xp too... Which is all good , plus you'll earn some horde cards along the way. 🙂
I got him after 2.5 juvie horde games