I think its less champion vs troops than stacks of doom vs everything else.
IMO Its ok to have the killer hero, all the best items/spells/etc that can take out a whole stack of mediocre troops solo. It should reach the point where it can solo most things, but a concentrated effort can bring it down or an inherent weakness.
It changes when you have 9 of them though, nothing except another stack of doom can stop it. Add mobility - cloudwalk, all mounted + tireless, etc and 1 stack can dominate. Which is what I usually do, until I make a 2nd 
Anyway, I wish there was a mechanic for prevention - at least for champions.
Exponentially increasing maintenance - Inherent magical abilities work poorly together, causing a rip in the space time continuum that sucks mana when 2 or more mages get together.
An EGO stat - each army can only have an ego limit of 10, and each level increases its ego stat by 1 (unlimited for 1 hero). So you could have a few weaker heroes working together, but as they increase in power you have have fewer. Till finally its a limited stack of doom with 8 archers and a melee defender hero. Which is still better than 9 heroes.
