Halo 3

Currently, the only way to get this achievement is in rumble pit on a mythic map. Once an infection gametype on a mythic map starts, if you are human then you will start with a shotgun and pistol and the infected will have no shield. One headshot with the pistol will kill the infected or a shotgun blast from close by. all it requires are two kills and you will get the achievement.

If you start as the infected, try going for the delicious brains achievement and wait till the next round and you will probably (although not definitely) start as a human.

This achievement will be easier to earn in the living dead double experience playlists.

This achievement can only be unlocked in rumble pit if all the players have the mythic pack, so i would advise playing with friends who also own the mythic map pack

the easiest way, and the way i got it, is to go into living dead playlist, it's on every couple of weeks or so - bungie picks them out at random out of rocket brawl, grifball, this and that might be it not sure...

Anyway, i got this on the orbital map, but it was quite hard. you have to 'camp' at the top of your starting point as a human and kill them at top using grenades or preferably your shotgun.

one map to definitly NOT try it on is the dead docks one, although you can try for 'delicious brains' achievement, as the zombies are invisible. although you can try it on these following maps, my method is advisable.

the maps you can get this on are:
-Dismemberment (Assembly)
-Dead Docks (Longshore)
-Andromeda (Orbital)
-Spooky House (Sandbox)

remember, you have to kill 2 zombies in 1 GAME. it does not have to be in 1 round, just 1 game.

As of August 2021, you can STILL obtain both infection-related achievements:

- Delicious Brains - On a Mythic map, infected 2 humans during a ranked or social match.
- Zombie Repeller - On a Mythic map, killed 2 Zombies during a ranked or social match.

Rumble pit is currently broken via playlist selection, but the infection game mode "Mmm...Braiiins" can be found on the 7 on 7 playlist. My session group went in with 7 individual players, but you can have extra controllers for guests or at least a few players to start the matchmaking search.

The Mythic maps are as listed; Be sure to pay attention to the pictures because map variants still count!:
1. Assembly - Dismemberment (variant)
2. Orbital - Andromeda (variant)
3. Sandbox - Spooky House (variant)
4. Longshore - Dead Docks (variant)
5. Heretic
6. Citadel

HOW-TO GET ACHIEVEMENT


It took awhile hours to finally find the "Mmm... Braiiins" playlist on the CORRECT map (ours ended up being a map variant of Assembly). If it isn't on any of the maps listed, you'll have to search again until it rotates to the right map. We got the right playlist 2 searches in, but it was on a base map so it didn't unlock achievements. GLHF!

EDIT:
As of August 30th 2021, the Rumble Pit Playlist is working again! I saw several Braiiins playlist on different infection maps so go for it!