Middle-earth: Shadow of Mordor - Game of the Year Edition

Start with the Bright Lord DLC:

When you kill Uruk captains/warchiefs, they will drop a rune (weapon modifier), which you pick up with RB. The level of the dropped rune is based on the following:

- ____ pts (1 point equivalent to Uruk's power level)
- 2 pts (for being Uruk warchief)
- 1 pt (revenge kill)
- 1 pt (knowing the weaknesses of the target)
- 1 pt (for exploiting target Fear/Hatred)
- 5 pts (issue target a death threat)
- 1 pt (killing said target above)

Edit:
It should be noted that when doing the first domination for intel it should be a regular Uruk, not one with the intel marker, because you don't want to know their weaknesses or it adds another point and you get a level 26 rune.
Thanks to Erod550

Easiest way is to start the Bright Lord DLC and dominate an uruk (+) and select gain intel. Next identify a "shadowed" captain till you get one that is power level 19, restart if you don't, it's faster. Dominate another uruk and issue a death threat (X) on the level 19 captain. This will bump their power level to 24, remember that a death threat will also add another point to the rune if you kill them thus level 25.

The best way to proceed to kill this captain once you've made him a target is to shadow mount (+) on a commonly spawned Graug located in the cave on the far left side of the map in the Durthang Wastes. Ride the Graug over to your target, cutscene, and press Y to eat him. This will force him to drop a rune at level 25. Or you shadow mount a Caragor (big cat), which are everywhere and bite the target. These are your better options.

If for any reason you are having trouble getting the Graug to spawn, just keep reloading the game (may take a few tries), but I found this location to be the most reliable.
https://screenshotscontent-t5001.xboxlive.com/xuid-253327480...

The reason this is the fastest way is because you don't risk getting overwhelmed by his buddies and dying (revenge target) and you wont accidentally discover his weakness or fear/hatred. Plus its way more fun.

Summary,
1. Bright Lord DLC
2. Dominate a regular uruk for intel to find a level 19
3. Dominate another to issue target a death threat (level 24)
4. Run West and shadow mount Graug
5. Ride Graug to target, press Y to eat
6. Dismount and press RB to pick up level 25 rune.

Hope this helps :)+)

I followed a few guides for this but found myself struggling, but I found an easy way to get a level 25 rune. On the Bright Lord DLC the enemies have power levels over 20 most of the time. Luckily I came across a Level 24 so I only had to either know his weakness, use his weakness against him or be his revenge target.

In order to do this, you must make a level 25 rune. This can be done either in the base game where by killing a level 20 captain plus the bonus (where level 25 runes are the max) or you can do this in the Bright Lord DLC.

Doing this in the Bright lord DLC is simplier, but beware, getting over 25 (26 to 30) will NOT get you the achievement.

The way I did it was to get a level 22 Captain, let him kill me (which made him 23 when i came back and revenge killed him)

Make sure you do NOTHING that uses his fear (so if he is weak to combo finishers, chose another)

When you come back to him, make sure you have your ring power max and simply keep combo finishing him with the fire swords to death.