Everyone is over complicating this.
Here is my method which is 100% guaranteed to work.
You can do this in matchmaking legit, or by boosting with extra controllers/with friends, just make sure you play on the map SHATTER.
Once the game starts, go to the areas of the map where there are "gravity lifts", they can be found close to the default spawning area. They can be found on the lower sections of the map and they shoot you straight up to the next level (kind of like an elevator).
All you need to do is get on the gravity lift and STAY hovering on it! While you are hovering on it, have a friend stand at the top level where you would normally exit the lift. Simply melee your friend 2 times, and BAM, achievement unlocked!
EDIT: SeraphimNeeded has pointed out that "If your friend is floating in the lift as well, you'll also get the other man cannon related achievements."
Use his idea for:
The ODST achievement in Halo 4 worth 45 pointsIn Crimson DLC matchmaking, kill a player while airborne from a Man Cannon. The Clay Pigeon achievement in Halo 4 worth 74 pointsIn Crimson DLC matchmaking, shoot an enemy who’s airborne from a Man Cannon.
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