After spending an annoying amount of time trying to get a level 25 rune, I've compiled information regarding how the rune calculations work. This also works for the level 30 rune achievement, just have the Warchief be level 25, rather than 20.
The rune you get from killing a Warchief or a Captain is tied to their level + several modifiers. You kill a level 17 Captain with no modifiers, you'll get a level 17 rune, etc. The modifiers are as follow:
+2 for being a Warchief
+1 for having a Death Threat against them
+1 for knowing their weaknesses
+1 for killing them with their weakness (must be an insta-kill weakness, very difficult to get a level 25 Warchief or Captain who still has one)
+1 for exploiting a fear/rage
+1 for being a revenge target (they've previously killed you)
The fastest way that I've found is to get a Warchief to level 25, have him kill you, know his weaknesses, and have a death threat on him. Now, the important thing to note is that if you kill the Warchief, it's likely that you'll get an Epic Rune rather than a level-based one. The way around this is if you have the Warchief die by means other than yourself, such as a wild caragor/graug, or a branded Uruk (I used the Warchief's bodyguards to accomplish this), he will be guaranteed to drop a level-based rune. You can still get a level-based rune by killing him yourself, but why take the chance? Hope this helps anyone who had issues like myself.