I tried the other solutions but they didn't work for me. What I did do though was very similar to the others. I leveled up a captain to 20. Gain the intel on him. I let him kill me for the revenge bonus. I also exploited his fear/hate during that fight which happened to be fear of defeat (at low health he gets mad and starts to heal).
I then went after him again and branded him. Next I went for the war chief that he was a bodyguard for. I killed the war chief and my branded captain took his place. I then summoned my captain, now war chief, by going to the mission marker on the map. I grabbed him, dominated him, gained intel on some random orc and then killed him. This worked on my first try and dropped a level 25 rune.
TL;DR
Before War Chief:
Level 20 captain. +20
Intel on him. +1
Let him kill you for revenge. +1
Fear/Hate exploited during that fight. +1
After doing the above, brand him and make him become a war chief.
After War Chief:
Dominate and kill him as War chief. +2