Before you pull all your hair out trying to find Captains with the right weakness to try to combat brand using A+B after you get a high hit streak like you would with normal Uruk, there is a much simpler way to unlock this achievement.
Bring up any Test challenge, preferably one where you start with intel such as Test of Wisdom. Pick and choose Captains by proximity to your current location and mark them. Use your wraith vision to locate them. Try to isolate your Captain as best you can once found so you don't have other Uruk interfering. You simply need to fight until the Captain's health bar turns green, grab him with RB and hold on. You'll have the option to brand with B while holding him. Simply hold B until the branding bar is full. Repeat with twenty different captains and you'll unlock this challenge.