Medivh is nowhere near as powerful as Sargeras. To even say something like that says you don't know much about the lore. He isn't even as powerful as most of the Dragon Aspects either. Maybe as powerful as Ysera since she is the weakest one, but we really don't know.
Generally people think Medivh is much more powerful then he really is. Yeah he is powerful, but people give him waaay to much credit. To this day Azshara when she was still a Night Elf 10,000 years ago, is still considered to be the most powerful mage that has ever lived on Azeroth. That means she was more powerful then Medivh. Medivh is still stronger then the majority of people on Azeroth yeah, but not as strong as people think.
Part of the problem is Blizzard has never given some type of list as to how strong the major lore figures are. So basically all we have to go by is person a beat person b so that means they can beat person c because person c fought person d and didn't win because person d beat person b.