For this Achievement, you need to earn 21 stars as Harley Quinn, 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 Harley Quinn:
-Harley does not seem to have many combos takedowns availabe to her, but she does have Mayhem Mode. Get special combo knockouts(
) to build Mayhem Mode, and then Activate Mayhem mode by hitting
. This will allow you to get FOUR special combo knockouts just by hitting
. These will have a x50 multiplier on them, so use it when you have a high combo.
You can play the following challenges to get 21 stars(my stars are in parentheses):Combat Challenges:
-High Interest (I had a hell of a time with this one. I had to have a Freeflow perfection, and activate Mayhem mode with a x40 combo to get all three stars. (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)Predator Challenges
-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.