To easily obtain the Overkill Medal, custom, social, or ranked, you should steal kills, or obtain the noobiest arsenal.
If you want the achievment (without cheating), go into a ranked Lone-wolfs match and just try your best.
The way to cheat for the achievment, go to ranked FFA, and have language set to the last language, hit x in matchmaking lobby, and select the last option, then start searching and youll find lots of people that want to gain achievments, non-legitimatly.
Something else is that when you play the "Achievment match" don't try to play it on Chinese, you won't find any one at all.