Play against a human.

So, in a game against a human player, is level 15 roughly the ceiling for a hero's development?

It depends on the map. We played "Internecine War" - it's too empty, with few neutral creatures and resources. "Shattered Alliance" is richer in this regard, and development would be faster there.

I played Inferno as a Warrior against a Necromancer. It was decided by the opponent's mistakes... in the end, he attacked my main hero, thinking it was a secondary one, and didn't deploy his entire army, and he didn't have time to escape. And I made a serious blunder at the beginning (I won't reveal my strategy yet, I plan to play the entire Inferno tournament), but it worked out for me in the end :)