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Play games, when you come to Defiant maps (Condemned or Highlands) now is your chance.
I'd advise Condemned, it's got tighter spaces that make grenades and explosives more lethal which make people use armor lock more often.
You want to toss a grenade or just wait for someone to go into a long armor lock. Maneuver behind them as best as possible, but stay back a little so your shield doesn't get EMPed. As soon as they come out of it, move in and hold the bumper. Hopefully you get the assassination and not a beatdown.
Obviously it's better if you're in a 1 vs 1 duel so interlopers don't kill you as you wait, or you can try to come up behind a different 1 vs 1 duel. A lot of times when about to die, someone will toss a grenade and the other guy will go into armor lock if they have it. And since people tend to stick with their loadouts, keep looking for the same player once you know they're using it.
A little luck, a little skill and looking for the right scenario.
EDIT: It's somewhat complicated in that while in armor lock, you can move your cursor around the screen so that you instantly turn as you come out of Armor Lock. This can frustrate getting an assassination vs just a beat down.
EDIT: Reach is now backwards compatible, I have not verified myself but you seem to no longer need it to be an assassination, a ninja will work as well. Melee from behind, not the animation.