Hey Folks, NOTE: My achievement popped when I completed Azrael's Atonement, which would have been 26-stars for me. Since the same thing happened to my Nightwing achievement, I am going to assume that one (or more) of the previous challenges did not count for Catwoman, as I went straight down the combat challenge list.... Please comment if you have the same issues.
For this Achievement, you need to earn 21 stars as Catwoman, which is actually quite simple with all the AR challenges that Arkham Knight has to offer. Personally, I find Catwoman to be on the slow side when it comes to combat, but I was still able to get this achievement easily. You should be able to unlock this by doing the Combat or Predator Challenges. You do not need to 3-star the challenges, just accumulate 21 total stars. The challenges that have Batman and Catwoman teaming up DO NOT count for this achievement.
Combat Reminder/Hint: Environmental Takedowns are a x100 multiplier, so use them if available on a high combo. Also, for getting a freeflow perfection(never breaking your combo) you will score your strike counter multiplied by 100 at the end of the round. (22 combo x 100 = 2200 bonus)
Tips as Catwoman:
-Catwoman does not face any giant thugs, which can be good or bad, depending how you build your combo multiplier. However, it seems like for every giant thug you don't fight, you get an enemy with a riot shield.
Also, Catwoman can break the electrified enemies via her whip (LT). Thanks to otaku1412 for the tip!
You can play the following challenges to get 21 stars(my stars are in parentheses):
-Shark Bait (2 stars)
-Precinct (3 stars)
-Flying Grayson (make sure to use environmental takedowns) (3 stars)
-Feline Frenzy (3 Stars)
-Newton's Cradle (3 Stars)
-Catwoman's Conundrum (3 Stars)
-Teen Titan (3 Stars)
-Combo Master (3 Stars)
-Azrael's Atonement (3 Stars)
-Smash and Grab
-Revive and Shine
-Under the Pale Moonlight
That's a total of 63 Stars, so play at your strengths, and don't stress out too much.