Medivh had to tangle with Sargeras' presence in his mind, and kind of held out against him.
He did not fight Sargeras one-on-one. Sargeras would have destroyed him easily.
If you want an example, try Bolvar and the Lich King... the Lich King tormented him for quite a while, trying to get Bolvar to abandon the light... but Bolvar did not, and held out against the Lich King's torture. However, I'm pretty sure the Lich King would own Bolvar in one-on-one combat without too much trouble.