As part of the story you will eventually go into the steel mill. Keep on going through the steel mill until an Enforcer comes and fights you with a group of thugs.
What you need to do is to ultra stun the Enforcer (with B,B,B) and the spam the X button to attack and counter with Y when an enemy is going to hit you in the middle of your combo.
When I did this my combo was 76x which was more than enough for this achievement (50x combo) so you could have a margin of error before you reload your checkpoint to try again.
If you happen to have the Deathstroke add on pack that gives you the challenge pack 1vs100 load it up and put on the perk to punch through armour shields and taser-batons. When you start the match start your free flow and get a 50x combo. If you don't have the challenge pack the other method works just as well.
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