Grom is the superior fighter and at least as strong as the Mountain, he takes it, easily. I also think that SoIaF is rather low level fantasy, powerwise, compared to Warcraft, which is epic level. Grom would be something aroung 16th to 18th level, while the Mountain would be somewhere near 7th level in D&D terms.
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I think it is a good system to rate chars in general. Originally it was mostly level 1-30.
At level 9 you started to become a significant force in your world. Level 18 was kind of epic, beginning for some Archmages in some settings, like Greyhawk, in others 20-30 was the max and almost a god tier. Though goods could have levels till 40 in 2nd Edition. Now in the 5th Ed some things changed and I would do it like this. Max character level 20, even multiclass, like Warrior/Wizard. Once someone goes beyond level 21 he becomes basically a god. Though Superhumans could still be just level 9 and be challanging to chars of level 24 with high stats and special abilities.
As for ability Scores, you have Strength, Dexternity, Constitution, Intelligence, Wisdom and Charisma, 8-11 is the average, the max for mortals is 20, though you can see it like this, 18 Int would be an 180 IQ, which is highlY unlikely to have. Scores of 20-30 are reserved to gods and godlike beings. You can differentiate the scores further with boni to rolls, so someone with strength 30 and a bonus of +10 is not as strong as someone with an strength of 30 and a bonus of +20.
Anyway, my appraoch to the levels:
Level 0 Apprentice (in our World and the fictional)
Level 1 2 Standard Worker
Level 3 4 Years of Experience
Level 5 6 A Real Professional
Level 7 8 One of the best in the World
Level 9 10 A Legend in our World and the beginning of the Epic ones in Fiction (Comic/Books/Movies)
Level 11 14 Legendary Heroes, Superheroes (Street Fighters), Jedi Masters etc.
Level 15 16 People like King Arthur, Aragorn
Level 17 18 Archmages, Superheroes like Dr. Doom, Akuma etc.
Level 19 20 Quasi Powers in D&D, Elrond, Galadriel
Level 21 23 Demi Powers in D&D, Son Goku, Dr. Strange, Maiar, Ingwλ, Low Heralds like Hercules
Level 24 26 Lesser Powers in D&D, Mid Heralds in Comics like Hulk, Maiar, SSJ Level 4 Goku or Beerus.
Level 27 29 Intermediate Powers in D&D, High Heralds like Thor, Silver Surfer and Trans beings like Thanos, Valar
Level 30 Greater Powers in D&D, Manwe (Valar), Skyfathers like Odin, Lady of Pain
Level 31 Cosmic Abstracts like Chaos and Order, Overpowers in their Realm (Lady of Pain, Ao)
Level 32 Cosmic Abstracts like Eternity or Galactus (Universal), Overpowers of Planes (Asmodeus)
Level 33 Cosmic Powers like the Infinite Gauntlet or the Heart of the Infinite
Level 34 Multiversal/Omniversal Powers like the Living Tribunal, Spectre, Michael and Lucifer, Tom Bombadil, Anti-Monitor, Multiversal Overpowers.
Level 35 Ultimate power of a fictional Setting, Presence for DC, Io for D&D, Illuvatar for Lord of the Rings, The one above all in Marvel
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See it this way. Someone with level 9 would be a leader of one of the major houses, a great general or even King. Jaimie would be a level 10 fighter for example, Arthur Dayne a level 14, like Barristan now or Ned Stark before his death, or maybe 12. This is not only about skill but also leadership etc. I could see the Viper also as an level 10 Fighter, the way he was toying with the Mountain.
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