This achievement can be a little tricky because it has to be done in the Anniversary playlist in matchmaking. For those of you who do not know, a beatdown is when you melee a player on the opposing team from behind and get the instant kill without going into the assassination sequence. To do this, you have to lightly press the melee button, otherwise you will go into the full assassination. For the cheevo, you have to do this while you are SIMULTANEOUSLY pressing the button for the jetpacking armor ability.
The way I did this was to go with a friend into Anniversary FFA and I had him stand on a ledge where I was able to jetpack up to him and lightly tap the melee button for the beatdown. The cheevo popped almost instantly.
Also to be noted is that in the Anniversary playlist, the Anniversary gametypes do not let you use armor abilities. You must choose a regular slayer or objective game in order to do so.
edit: Another maybe even easier way to do this. I did it in doors so that i was floating along the roof while holding jetpack but still in range for punching my friend in the back. Credit goes to theredjacket.
edit: An even easier way to do this when boosting go up behind someone, crouch, start jet packing and immediately melee fast(make sure not to hold or you'll assassinate). This method makes it possible to get the achievement almost anywhere on a map. Credit goes to neb86.