According to my experience, Primarch power level varies greatly. Namely, it depends on factors such as author, book, situation, and simple entertainment.
For example, most of the Primarchs almost casually massacre whole squads of Space Marines at once, but our 'Avenging Son' almost get killed by an Alpha Legion hit squadㅡeven if he is completely unarmed and defenceless, I think it is a little absurd, especially when compared to the Night Haunter, who take a active role in the same novel. In the previous HH series, Primarchs are portrayed consistently virtually impervious to 'mere' boltershall variants and withstand a direct hit by tank guns and aerial bombardments without a scratch.
Another great example of elastic power level of Primarchs is the Lord of Raven. In the previous novels(such as 'The First Heretic' and several of short stories) Corvus Corax not only slaughtered whole army of Gal Vorbak and annihilated most of them but also rip apart Predator tanks and endured all of the punishments.
Whereas in novella Corax: Soulforge, he almost get killed by 3 Defiler(if there is no air support, he would certainly have died) and struggled with a mere mutated Chaos sorcerer, and he was not even particularly powerful or renowned one.
If there is any solid consistency in Primarch power level, it is that even if Primarchs in their weakest state, he could slay full-blown veteran Astartes like they are 5 year old child. Then again, there is exceptions like Kor Phaeron, 13th Primarch again almost get killed by a mere half-Astartes, and only through exploiting Master of Faith's chronic hubris Roboute could defeat himㅡand at the same time, Leman Russ was practically inviolable almost all of the Warp magick except ones wielded by his brother equal and future nemesis, the Crimson King.
Last edited by Ecumene; 04-16-14 at 01:41 PM.