Based on this episode alone, is it reasonable to presume that Granamyr has power and knowledge beyond Greyskull?

He put He-man in some kind of mystical bond, that he couldn't break. (it was no effort when he did so). He knew how to reverse the spell that made Man-At-Arms into crystal, and referred to that spell as it was child's play to him, and yet it was beyond The Sorceress's knowledge.

So here is one question, if this Granamyr was to use his full potential, could he actually be defeated?

The way I see it, in that particular episode, comparing He-man to Granamyr is like comparing The Silver Surfer to Galactus. (a well written Galactus that is)