He needed the help of a daemon though. And as much as it pains me to say it, Russ he (supposedly) killed two other primarchs, c'mon hes the Executioner.
He needed the help of his legion I'm sure (Angron and El'Jonson wrecked him one on one), since supposedly the missing primarch's legions were absorbed into the Ultramarines
Man Magnus was a badass when it came to the understanding of the warp, even so he still got outplayed by the Dark Gods, Rogal was a really smart tactician and his chapter defended terra like a boss, Sanguinus was the pretty boy even tho he went WWF on that demon defending the eternity gate, Alpharius and Omegon were black ops kings, I can be here all day and talk about the uniqueness of every primarch, imo is between the Lion and the Wolf, with Rogal pretty close, I did not mention Lorgar or Horus, because they were both a bunch of spoiled brats.
