Medivh was actively fighting Sargeras' influence up to the point that Khadgar and Lothar killed him, as detailed in both The Last Guardian as well as Chronicle Vol. 2. Medivh's last words before his death were to thank Khadgar and Lothar for freeing him, before his soul, freed from Sargeras' taint, was sent to the "astral plane," presumably what we now know as the Shadowlands (Chronicle Vol. 2, pg. 134). The same is true of his duel against Aegwynn, detailed similarly in Chronicle Vol. 2, pg. 118.
Argus wasn't fully realized, no; but again he also received direct empowerment via Sargeras - and even in his unempowered form was fully capable of ending all life in the physical universe. We did have the Pantheon's help, as well; but they didn't really actively aid in the fight - up to the point that Eonar resurrected us (after Argus kills the entire raid in a single shot) they'd taken a pretty passive role doling out a handful of buffs. Devos really only wounded Kyrestes, and she also had been empowered by Zovaal - so she wasn't really acting just under her own power, either. I also think the heroes did most of the heavy work in defeating N'Zoth as well, so I maintain your comparison there is still flawed.