In order to get a level 25 Rune, you must kill a level 20 Uruk/Orc with + 5 levels. You get + 1 level from making him a revenge target, + 1 for intel to know his strengths and weaknesses, + 1 for activating his Hate or Fear, + 1 for killing him with his weakness (ex. Combat Finishers), + 2 for him being a Warchief, and + 1 for sending a Death Threat (read the section below on more information about Death Threats.) Now I know this adds up to 7, but I'm not so sure about knowing his strengths and weaknesses and kill him with his weakness. I'd suggest you do ALL OF THIS anyway.

It is true that using a Death Threat does greatly increase the chance of getting an Epic Rune but ONLY if YOU YOURSELF kills the Uruk. You can send a Death Threat, which will increase the level by 1, and then have SOMETHING THAT IS NOT YOU kill it. So either have a Branded Uruk kill the Warchief, or have some animal kill it. Special thanks to dAn De lIOn giving me this information.

Step 1: Find a Worm (Green targets on map) and use the intel to know the strengths and weaknesses of the targeted Uruk (preferabbly a HIGH LEVELED WARCHIEF but it doesn't matter). He can be either a Captain or Warchief but it's easier if he's a Warchief. THIS IS ALSO VERY IMPORTANT. Make sure he has a Fear or Hatred and Weakness of something. If he doesn't, just ditch that Uruk and find one that has a Fear or Hatred AND Weakness. Once you find one with a Fear or Hatred (For example, fear/hatred of fire, caragors, etc.) You're good to go. Knowing the intel adds a point.

Step 2: It does not matter what level or rank (Captain or Warchief) they are IN THE BEGINNING. The closer they are to 20, the faster it'll take to get the rune. And just make sure in the end, the Uruk is a Warchief. If they're level 16, just die 4 times to them to get them to 20 and you also complete the task of making him a revenge target, which adds a 1 point. If it's a level 20 Warchief with a Fear or Hatred and Weakness that you found immediately, YOU MUST DIE TO HIM ANYWAY.

OPTIONAL Step 3 (IGNORE IF YOUR URUK IS ALREADY A WARCHIEF): If the Uruk is only a Captain and you did these steps, do not fear. You can just find out who he's connected to (what Warchief and what other Bodyguards he has. In order to make this simpler, you can send a Death Threat to THE WARCHIEF which means when you fight him, the Warchief and all his bodyguards will come out. Kill the Warchief and the other bodyguards THE ONES THAT YOU AREN'T MAKING LEVEL 25 and then Uruk that you're making level 25 will become a Warchief, adding 2 points.

Step 4: At this point, the Uruk should be a Warchief, should have killed you once, be level 20, and you should know his strengths and weaknesses. You can either let something else kill the Uruk such as animals, explosives, or branded Uruks/Orcs or you can just kill him yourself. Letting someone else that's not you kill the Uruk will ensure a Regular Rune will drop while killing it yourself will only have a small chance of an Epic Rune to drop. I killed the Uruk myself only cause it's generally faster than having someone else kill it so you can choose which way. What I did was in the beginning, I immediately set off his fear or hatred, in my case, hatred of fires. Then after that I just kept on getting combos so I can do an execution move on him. If he's not affected my execution moves, you can just spam Wraith Flash (default bind R) until he's low enough for you to grab and shank him, or until he dies.

Step 5: Once the Warchief is dead, you should get the level 25 Rune.

In order to get a level 30 Rune, it's pretty much the same process as getting a level 25 Rune. In order to do this, you're going to need to do it in the Test of the Ring challenge. Pretty much do the exact same thing as you did to get the level 25 Rune, except this time, your target must initially be a Level 25 Warchief/Captain. Quite simple.