You should check out Kexol's guide for what medals to get, but I think Halo CE is by far the best playlist to do it. 4 players max, and it also can be done with 2 people with an extra controller each. KOTH comes up in this playlist a little less than half the time.
In 2 groups of 2 search in the playlist at the same time. Communication is key, as the party leaders need to know when to stay in the lobby or back out. How you determine that is if you see the same text on screen for the 2 parties at the same time. For example: found players and connecting to players. If it's not at the same time, back out.
The way I do this is by getting double grenade kills on people inside the hill. The ones who died, has a person on there team get double grenade kills on the first team. Switch teammates that get the kills, rinse and repeat. This way you'll be boosting Double Kills, Grenade Kills, and KOTH medals at the same time.
Spend as little time in the hill as possible, to keep the game going as long as possible.Best method I've come up with, I hope it helps you.