I've run it twice now, and both times the players felt challenged and that they barely survived. That's what I think is an ideal running of the scenario.
The last time I ran it, Krune got cut out of his sarcophagus by an adamantine axe-wielding barbarian. He was unable to act before he was almost dead (managed to disable all runes). He used dimensional steps to get away and then Phase Door to hide in a wall. He summoned bralanis to heal him, but the party had True Seeing going which allows seeing into ethereal. He was mostly healed when a successful Dispel Magic popped him out. The party had a couple wands of Dispel Magic at high level thanks to a particular scenario, and they rolled well. His Black Tentacles suffered the same fate. He readied an empowered maximized horrid wilting for when the last party member ran into the room. It went off and made them all pray that the barbarian's attack would finish him off. It did.
Die rolls can make a big difference.