I beat it down to 14% but you could probably stop around 20%
I then turned away so i would not melee it and nuked the adds while doing this.
While doing no damage to the mob after about a minute it suddenly became captured and appeared on my cursor
So what I am thinking is you can beat any of these Ranger epic mobs down then just wait patiently
a few minutes and you will capture it. Make sure you have all damage shields/ procs off.
Don't kick it either.
Hope this info helps

