Once I reached the end of Act 1 and got the ability to Dominate and Recruit enemies, I did the Bruz mission. After recruiting him, my next recruit I was forced to set as a body guard when Bruz arrived and explained how to do it. Then my next recruit I was forced to send to fight another captain as Bruz again explained how.
In doing this, I was set a waypoint to go and join my recruit in the fight. I went straight there and started the fight. I joined in by taking out the surrounding enemies, but I ignored the captain. My recruit had fire arrows, so his first hit blew something up doing half damage, then the captain continued to burn until they died. This unlocked the trophy for me.
You can perform this action any time after you have been taught how to do it. So if you don't get it first time, don't worry, you can try any time you want as long as you have a captain recruited. Simply press Options and select Army, then highlight a recruit and press triangle to Command them and then issue a kill order on another captain.
It works best if either your recruit is higher level, or if they have something that the target is afraid of, like fire damage. You will have to have revealed the targets weaknesses to know if it is a good match though.