Halo 3

Get at least two double kills- 2 kills within 4 seconds of each other- on Avalanche, Blackout or Ghost Town. Relies more on luck (you have to find two opponents that are close to each other and kill both of them quickly). Keep playing and you will get this eventually.

You can get this achievement on a Mythic map as well. Just find two enemies and kill them within 4 seconds of each other.

Then, find two other enemies and kill them within two seconds of each other.

Grenades work great here.

I actually got mine playing Action sack. I waited till I got a Stab-out game.(Team Slayer with swords) and I stuck together with the team, and waited till there were 2-3 opposing team players coming for me.

It might take a few tries, but there will be plenty of opportunities. Should not take more than 1 or two matches on Stab-out.

Just be careful that you dont get assassinated while going for the kills.

I hope this helps.

This, and all DLC achievements can be done in the Team Mythic playlist (except the Delicious Brains and Zombie Repeller, which must be done in the Infection playlist, Tank Dropper and collecting the skulls, which must be done in Forge mode, or Killtacular which requires 5 opponents, either in a free-for-all or Big Team playlist). All DLC achievements will unlock on maps from later map packs.

Team Mythic requires 6 players, and maxes out at 8 players. You'll have to get 2 players together with 4 controllers each, 8 players, or somewhere in between. If you can't get 8 players and/or guests, you can do this with only 6; however, you may need to put up with randoms, but they will likely be willing to help. After all, who would still be playing this mode unless they were achievement hunting.

Other than that, I think this is a pretty straight forward achievement, but if you need more guidance, refer to one of the other solutions.

Surprisingly enough, I got this achievement without owning any of the Map Packs. So just get two Double Kills. It's fairly simple if you know what you're doing. I don't remember ever playing or buying any DLC for this game and I got this achievement.