An Imperial assassin droid loaded with programs of the greatest combatants in Galactic history and who's capabilities were specifically improved by Vader being superior to animated junk droids? That's crazy talk.
He was stated to be more of a challenge for Galen then Rahm was by Vader. Vader has no reason to lie. Kazdan has the feats to suggest he is. I don't see why you are so adverse to acknowledging his superiority. Given Galen was suggested to essentially be Luke if he had been raised by Vader I don't see a problem with that line of reasoning.
"He builds a replica of the Jedi Temple from debris gathered from an abandoned industrial quarter of Raxus Prime."
So he built it with his hands?