This is a missable trophy!
There is a place in the administration area, after you fight
. Joker will lock a door behind you and pretend to talk to Comedy Weekly magazine. Head forward and begin the fight with the thugs. You will need to use the Cape Stun>Beatdown on every enemy, with well-timed counters when necessary. I got my combo up to x116 by the end, so you have a little wiggle room at the beginning.
I personally found it easier to keep an enemy behind me, only because it seemed easier to time the counter from that direction. If you lose your combo, just restart from checkpoint and it will put you back at the door. Also, if your beatdown is about to end and the counter symbol flashes, it's ok to finish the beatdown - the attack from the enemy is interrupted.
In the process you will get these trophies as well, if you haven't already:
It should also get you this trophy as well, or at least put you well on the path to it: