On the other hand, many players (mostly F2P) don’t have Proteus (not to speak of Hel) and have to find another way to beat difficult stages.
What OP means is that one single hero (well, two if we include Hel, who hardly anyone has) can singlehandedly change extremely hard boss stages to an easy win.
I think giving some of the bosses full or partial resistance to mana effects would make ‘hard’ stages a bit more challenging and interesting. I suppose that almost all players who have Proteus have plentiful alternatives to beat hard stages without him (or Hel). True, there might be a few relatively new players that were lucky to pull him very early but haven’t had the time to build a bigger hero roster. However, they usually haven’t arrived at S3 and are still fighting their way through Atlantis.
Therefore, introducing such effects among others in S3 ‘Hard’ difficulty would hinder only very few players to move on. The vast majority, even those who use Proteus all the time, can easily find alternative strategies to succeed. If zhose extra effects are limited to ‘Hard’ difficulty, those who really can’t do without Proteus, could still complete all the normal stages.
I agree with OP that levels in S3, especially on ‘Hard’ difficulty, can and should be really challenging.
Didn’t leave Proteus out on purpose, but I can tell that a mix of 4* and 5* heroe (mostly vanilla) without mana controlers was enough to complete the first three provinces on both difficulty levels without any trouble and without using any other items apart from small heal potions and small antidotes So it’s absolutely doable without Proteus and without superstrong paywall heroes.