At the moment, this trophy can be unlocked by commanding your Warchief to attack any captain OR Warchief. This may be a bug but the solution is essentially the same.
Later in the game, after a story event, you'll learn how to brand Captains and command them. Either brand a Warchief (this is hard because many of them have an attribute that prevents this) or a Captain. If you successfully brand a Warchief, get him in a grab/hold, then "Dominate" and "Command". From this point, simply command him to attack one of his fellow Warchiefs.
With a Captain, you'll first have to Command the Captain to become a Warchief's bodyguard, help them pass the trial, and then command that Captain to betray the Warchief. Join the battle yourself, help kill the Warchief, and as long as your Captain survives, he'll become Warchief. At this point, do the same as above.
As soon as you issue the command, this will unlock. You don't need to attend the battle or have it succeed for this trophy.