Based on the Filmation canon, it seems safe to say he's the most powerful being on Eternia. When Man-At-Arms was crystallized in The Dragon's Gift and even the Sorceress had no solution, visiting Granamyr was the only option. That's saying a lot about his magical and intellectual faculties.
He also displayed he could really "clean house" (as He-Man put it in Diamond Ray of Disappearance) physically when he came out of his damaged house in The Return of Granamyr and, with only a wave of his hand, banished Shadow Wing.
I also recall his being able to scan She-Ra's thoughts in Darksmoke and Fire, which is another level of ability. It disappointed me that he couldn't muster the magic to send She-Ra back home at first, but it was a nice touch when he combined his powers with She-Ra's sword.
Am I missing any Granamyr episodes from the list below?
"The Dragon's Gift"
"The Return of Granamyr"
"Battle of the Dragons"
"Darksmoke and Fire" (She-Ra series)
Afterthought: It's a shame things didn't pan out in such a way that Granamyr could be featured on the MYP series. With a few changes in his appearance, I think he would have rocked the house in that show.