I think you need to separate 'challenge' and 'complexity'...

Challenge: Enemies hit hard and have abilities that need to be interrupted / baited... requires players to know their class (cooldowns, etc.) and play as a team.

Complexity: Enemies have confusing, hard to remember, and / or poorly telegraphed 1-shots... requires players to know to 'the fight'.

More challenge, YES! More complexity, no thanks.