You can increase the chance of obtaining an epic rune by killing high-level Warchiefs and Captains. Allow them to win during Captains' and Warchiefs' Power Struggles. You can allow them to intentionally kill you so that their own rank and level increases. Then, return to them and get a revenge kill. You can also threaten the Captains and Warchiefs to increase their level. You will then have to take out their reinforcements as well.