You will get this during Act I as it has to be done to progress but it can also be done before the mission. Either way, focus on what the strengths and weaknesses are. It is quite easy to overpower the captains this early in the game, if, like me you have completed miscellaneous quests and collectables.
Here is the story mission where you have to kill a Warchief, and I show you how easy it can be. His captains were all killed within 2 minutes, along with himself.