Overkill Medal (Halo 3)
From Halopedia, the Halo wiki
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.
It is best If you have some friends do the same thing as it can't be promised that some people will be doing the same thing.