Can't "Brand a Captain while in combat" even if he's on green. What am I doing wrong?

[MaxRavenclaw]

I bring the cap to low hp and try the combat brand, but it doesn't work. He just keeps fighting. Do they need to have some special weakness?

[MaxRavenclaw]

Turns out they need to be vulnerable to instant kills. Couldn't find anyone that I could instantly kill with combat finishers, but instant stealth kill allowed me to stealth brand a captain and get the achievement.

[Ardenian]

I didn't know that one can Combat Brand captains. I thought you have to grab them when they are on green and Dominate them the old-fashioned way.