My vision of the encounter is built more on being creepy than utterly lethal. My original vision is that the creature won't even engage you immediately; it waits for someone to move into the interior before it acts. It's less "BIG OOZE WAITING FOR YOU" and more "The darkness itself comes alive around you." Beyond that, the creature can't leave the darkness and if you get close to the edge will retreat to remain hidden in the center. Again, it's not supposed to be a creature in the darkness as much as it IS the darkness - so the goal should be getting out as opposed to defeating it.
Beyond this, I'd actually withhold its superior abilities - Cloud Drift and its action point - for use if it actually NEEDS them. This is a critical point where the tactics as printed differ. Essentially, I was advocating intentionally inferior tactics because it's NOT a wargame, it's a story. The scene is intended to be like the trash compactor in Star Wars IV or the Watcher in the Water in The Fellowship of the Ring - you don't fight it; you're surprised by it and you do what you can to get away from of it, getting out of the darkness where it can't follow.
So. First, I'd run it less aggressively. That's a flaw with the module as printed; I'm just saying now, if you haven't run it, it's not how I would run it. Second, if you get a character engulfed who really, simply, cannot escape, I would bend the rules to help if the PCs have a good idea. Personally, I'd let someone using a sliding power (the pregen artificer's Thundering Armor, the sorcerer's lightning and thunder powers) to push the darkness OFF of someone its engulfed. Per my original tactics, radiant attacks draw its fire, and I'd consider having it release someone to go after the enemy if it's struck with radiant damage. Then you have to figure out how to get the radiant attacker out, but hey, it is supposed to be a challenge.
And yes, its insubstatial keyword should be completely ignored by radiant attacks, so they deal full damage + an additional ten points.