First up, you must have Batgirls Matter of Family DLC downloaded to play as her in the AR Challenges.
For this Achievement, you need to earn 21 stars as Batgirl, which is actually quite simple with all the AR challenges Arkham Knight has to offer. 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.
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)
A Hint as Batgirl:
-Batgirl does have the destroy weapon ability, so use that to your advantage when fighting riot shields and stun batons.
You can play the following challenges to get 21 stars(my stars are in parentheses):
-High Interest (lots of riot shields to destroy on here) (3 stars)
-Flying Grayson (make sure to use environmental takedowns) (3 stars)
-Feline Frenzy (3 Stars)
-Newton's Cradle (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 60 Stars, so play at your strengths, and don't stress out too much.