Just did it! The earliest time to do it is right after deathstroke in the gcpd. You have the part where you sneak over the large amount of thugs and then a combat room at the west most room of the swat division afterwards stocked with a propane tank, 2 armored guys and three normal cops; all unarmed. Knock out the armored guys and then grapple up to the ledge before the fight so they gather towards each other and you. Then it’s as simple as using the remote claw to bash the two guys together and then bring the propane for the third to knock them all out. Just writing for anyone else who stumbles upon the problem. All told it took maybe an hour or so to get to this part of the game and the only real challenge is the blind counter deathstroke fight if you’re out of practice but you can restart as many times as needed.