Orcs can only reach as high as level 20, so to do this, you'll have to boost the level of the rune when you kill the Warchief.
Find a relatively easy to kill but high level Orc Captain that is a body guard by interrogating worms. You want this Orc to reach level 20 BEFORE he becomes Warchief because leveling up after may cause him to turn into a Legendary Warchief, which will always drop am Epic Rune upon death.
Either make time pass through the forge towers or let him kill you to level him up to 20. Once he hits 20, get ready to kill his Warchief. Make sure to eliminate any othrr bodyguards so that the captain you leveled up will become Warchief when his boss is killed.
The hard part is out of the way for the most part. Let the Warchief kill you so that you get a "Revenge" bonus when you kill him.
Even though he is level 20, you'll get +2 levels because he's a Warchief, +1 level for getting a revenge kill since he's already killed you, and +1 level for knowing his strengths and weaknesses. That's a minimum level 24 rune.
All you have to do now is kill the Warchief and make sure you use one of his weaknesses or fears. This will get you the last level. When he dies, you'll most likely get a level 25 rune, though there may be a small chance of getting an Epic Rune unfortunately.
Just be sure not to issue a death threat as this will almost certainly result in an Epic Rune.
Courtesy of Slider257, we've got a pretty amazing tip. Though I haven't tested it myself, I highly recommend that you give it a shot as this could potentially save you hours of effort if things go wrong!
Slider257 said:A great trick is to backup your save to a USB thumb drive. I did that before battling my leveled war chief so when he dropped an epic rune, I reloaded the save from my thumb drive and tried again. Took 3 reloads before I finally got it so just be patient.