Funny coincidence is that Marcurio killed J'zago today in my game. I was running through the Hall of the Elements in the College and I ran through one of J'zago's spell practices. Marcurio followed me and got hit by J'zargo and then it was on.

Both are at the capped level - J'zargo is level 81. It took a pretty long time, but J'zargo died and Marcurio still had almost full health.
The thing with Marcurio is he has great perks. He has Recovery, Regeneration and Respite in the Restoration tree, so if he doesn't run out of Magicka he can live forever. He also has the Magic Resistance, Mage Armor and Augmented shock perks, so even at level 40 (his max) he's much more powerful than J'zargo.