First, dungeon should require fixed classes of unpopular characters. Like feca+eni (classical hi-end dungeon setup), xelor, sadida and eliotrope. Without this setup dungeon will be almost impossible.
Second, boss should have invul, which can be removed only with some not oblivious actions: like pushing boss 20 times or surrending boss with summons.
Third, when boss health goes below 50%, boss starting to summon 1 mob every odd turn and making huge aoe every even turn. You can survive aoe only standing diagonal to boss in range 3-6.
Add some cool mob family with 20k hp tank, healer and burst dd who can kill any char in one turn (also they should revive each over with no-LoS spell) and new HARDCORE-THE-HIGH-LEVEL-ISLAND ready. Only for PRO-players.