Someone correct me if im wrong, but i dont think magic used against Spectre makes him stronger. What i do know and i believe he stated this was that as long as magic "EXISTS", one can never rid the Spectre.
As far as Spectres fight with Nabu, Nabu knew that he nor anyone else could defeat Spectre and he knew that Spectre would eventually kill him. What he ALLOWED Spectre to do was kill him sooner than later. Thats all he did. The only person who both Spectre and themself knew Spectre could NOT kill, was Stranger.
Just wanted to make that clear.
As far as Spectres fight with amped CM, yes Marvel was doing well and Spectre did mentioned that he was almost destroyed. With that said, one has to take what he said with a grain of salt. Why? Because of what was mentioned previously. As long as magic still exists, Spectre would never be entirely rid of. Not saying that magic couldnt damage or hurt him momentarily, but to say that CM would have defeated him permanently wouldnt be accurate at all imo. Hed come back.