A good way of racking up Double kills is to play Infection (Living Dead).
Depending on which Halo game the lobby chooses. Halo 2 and 3 is the easiest for getting double kills, since the pistol on 2 and 3 is OP and can 1 shot zombies if you aim for the head.
Good maps to play for getting loads of Double Kills are as follows....Halo 2Cadre
- You'll want to camp on the beach with duel wield pistols and a shotgun, there pistol ammo near the grav lift on the right side of the beach. Zenith
- You'll want to camp near the edge of the map where the Banshee spawns in slayer. The zombies can only come from in front of you and up the stairs on the left. There's pistol ammo at the bottom of the stairs.Stone Town
- Good spots to go are in the beach area, it's wide open with not lots of places the zombies can hide to try and pounce you. Also near the beach you can get on top of where the sniper spawns in regular slayer. Just jump onto the pillars and jump across the platform and the zombies have to jump up the same way you did.Halo 3Snowbound
- Camp near the plasma turrets on the right side of the map out in the open. When you run out of ammo walk outside the map to get killed by the turrets to respawn with full ammo.The Pit
- As you spawn turn right and grab the turret then go to where the active camo spawns and camp there with the turret. There will be huge boxes behind you so it will take the zombies ages to break through from behind. Isolation
- You'll want to grab the mini turret on your right from spawn then jump down into the area where all the fusion coils are.Ghost Town
- As you spawn you'll want to keep heading forward until you get to a ramp that leads to a small platform. There's a fusion coil on it that you can move to the bottom of the ramp to get a Double kill or two.High Ground
- You'll want to camp where the Mongoose spawns in regular slayer, inside the base and on top of the anti air missile turret. There will be lots of boxes and fusion coils in the main entrance. Use this to your advantage and wait for zombies to go near it before blowing it up.Last Resort
- Exactly the same map as Stone Town from Halo 2. Camp near the beach area and on top of the tall stone pillar near on the beach.Longshore
- You'll want to camp inside the building near where the Long Shore skull spawns. The building will be shut so you'll want to jump on the ledge and crouch jump to get inside the room. Once inside the zombies can only get in through the window. This will be harder for the zombies since they jump higher.Rat's Nest
- You'll want to camp outside on the cliff edge in the open. There's Fusion coils nearby for you to abuse on the infected.Standoff
- You'll want to grab the mini turret on the side of the map where the edge of the cliff is behind you. The turret will be on a mounted platform. Once you have the turret camp in the open as the zombies can't take cover and mow them down.
Overall this achievement isn't that bad if you're decent at Halo. I played infection loads on Halo 3 (Xbox 360) and on Halo Reach so hoping that my experience of playing this, will help other people.