The players were forced to fight their worst nightmares before being allowed to even see the Id Fiend. So, one by one, the players were forced to make hard DC rolls (level 1, they were making DC 19 rolls). I didn't set any 'primary' skills, so they could essentially use their best score to get away, dodge, or otherwise resist their own nightmare. Some of the players ran away from slavers, some avoided a gigantic worm that came to eat them, others fought defilers, templars, etc. Players that failed to make their check grant combat advantage (are considered Stunned) to the id fiend... he happily gobbled up the opportunities at the beginning.
Inevitably, player will make successful checks. Once a player makes 2 checks, they break out of the nightmare and saw the real threat: an Id Fiend. Then the real fight begins.
The beauty of the encounter was that the players come out of their trance in a random location. They come out one at a time. One on one, the Id Fiend will win - and he was clobbering the first two players that woke up. Then, a bunch of characters started making their checks, and the tides of battle quickly turned.
... in the end, I tried to get the Id Fiend to run away... but he got taken down. Oh well
I remember talking about doing an encounter like this before. Never actually put it into practice until recently. Encounter was easy because the Id Fiend could hit the broad side of a sand dune
Since the ID fiend shifts a lot, slides the PCs, immobalizes them, and knocks them prone, consider using those to your advantage. A low cliff edge or bridge (10' drop) and there's a chance anyone could be pushed over the edge. Consider also a zone: something like a vent of scalding steam might go off in particular squares randomly or each round with a blast of damage. Defiled terrain could give Arcane/Primal PCs a penalty to their attack rolls while they're in that zone. That would take advantage of sliding, shifting, being prone, or immobilization.
Consider the PCs' strengths too. What could they use to their advantage? Dyamic terrain elements can be the most memorable. Assign the bridge or crumbling columns HP and defenses and a vulnerability to force/thunder, and keep track of any bursts or blasts that end up affecting those elements. A solid thievery, athletics, or dungeoneering check could be used to make the scenery crumble. Some quick thinking on the part of the PCs could box the Id Fiend in with rough terrain from crumbling walls or do extra damage by knocking the brige down on top of it.