For this Achievement, you need to earn 21 stars as Red Hood, 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 Red Hood:
-Red Hood does not have the destroy weapon ability, and it seems like there is an endless supply of riot shields when it comes to his challenges. Make sure you can dodge well enough to keep your combo going.
EDIT: As pointed out by Pr0style NL, you can also press
at any time to kill and enemy at any range.
You can play the following challenges to get 21 stars(my stars are in parentheses):
-High Interest (lots of riot shields on here, make sure to use environmental takedowns) (3 stars)
-Flying Grayson (lots of riot shields on here, 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.