The easiest way to this i found was on the 17/18 mission or whatever you have to brand all 5 of the warchiefs, so when you finish the game and all 5 of the warchiefs are under your command just meet them and kill each of them until you've killed all 5. However, do NOT die or advance time inbetween killing them all other wise you have a high chance that a captain becoming a warchief.
If you have already completed the game and lost the wachiefs or something, just brand some captains, command them do an initiation to a warchief and then force them as a bodygaurd to betray their warchief.
Or you can brand the ALL the bodygaurds of the warchief and command them to betray the warchief which will result in one of your branded captains becoming warchief making it easy to kill the warchief.
Rinse and repeat until the warchiefs are dead.