Why not just introduce a Training/Practice Run Mechanic so people can learn a fight?
I think one of the bigger issues is people just not knowing what they are doing, when you go in blind you dont know the mechanics, so it can also effect your DPS because you are worried about other things.
Gatekeeping isn't really a solution as it would probably just kill interest in doing content if you have to jump through a bunch of holes and then complete instances in order to do something. Heck, even if they did do that, its not like it guarantee's a clear. Other factors take place for every player.