I enjoy the challenge of H40, but I think that the amount of bosses they have to throw at us to make it difficult is exemplary for the power creep of players and powers.
There is a lot of discussion about what is OP, but personally I would remove any form of cooldown reduction to make abilities situational and not spammable. That would also take away the power from lots of strong abilities such as gold toughness, bubble, venting shriek, taunt, etc. In addition it would make it easier to balance abilities by modifying the cooldown.
That alone should make Havoc much more difficult without requiring to throw multiple bosses at you body blocking you in tight spaces.
Side Note: Almost all my runs that end prematurely do so due to multiple boss spawns.