Start up a Treasure Hunt game. (private matches do not count)
You'll only need 1 other person to unlock this achievement. Killing the same person 5 times, will do.
There's a few different traps on each map. (Fire, Arrow, Pressure, Electric, etc) Each trap has 1 or 2 glow discs next to them. Shooting the disc, activates the trap. Fire traps are located on walls. You'll see fire burning inside a hole with a glowing disc directly above it. Have your partner stand in front of it and shoot the disc. A flame will shoot out, killing him. Make sure you're not standing in the flame's path, before you activate the trap. If you happen to get a map that doesn't have a fire trap, just quit and search again. Stay away from the cold-weather maps and stick with the Egyptian or Mayan desert-looking maps.