Make a set up split as in "Veteran group" (only allow person who clear that dungeon/boss once before to tick it) and "standard group" (anybody can choose this choice, but the newbies are force into this).
Now, those who don't want to keep losing primal hard mode due to 1-2 person that is new to the fight and rest in peace. While those who only want fun and not care speed or effective can always go for choice 2, usually new people won't want to speed run first time dungeon anyway.