I'm surprised that anyone would consider leaving the parties 11th level wizard and 11th level cleric behind. Seems like an effort should have been made to get them to come along as NPC etc or a side adventure ran until their players could attend.
While most of my gaming background w/D&D is 1st ed AD&D, I do play 3e, I've never played with a group that would consider going into a Great Wrym's Lair without a wizard & a cleric along in either version of the game. This sort of thing requires the whole party to be sucessful not 2/3's of the party.
One knows at the very least the magic user can do 7th level spells. This should IMO send up a red alert that the party should have a wizard of some sort along. If this means eating goat and backout of the dungeon to go back and get the magic user so be it. As a player, I'd want the party wizard to either memorize Anti-Magic Shell to have a schroll with it. Any 11th level wizard in 3e should at least have schroll on with AMS on it. As it can be used to shut down the other sides magical attacks/spell casting. This sort of thing was always a given within our 1st ed groups at this level.
As for the combat encounter, when the Prismatic Wall went up, the parties Mu might have had a chance to knock it down with an AMS since the AMS doesn't allow magic to function with in it. That would have nullified the spell attacks etc direct toward the party at least for a time. Likewise, some sort of spell to determine what was behind the door is pretty reasonable at this level of play IMO. A wizard eye, augury, clairvoyance, esp etc.
Or at the very least the party wizard might have cast wall of force or iron between the ice golems and the PC to try and protect him from getting tossed into the Prismatic Wall or the "most excellent prismatic wall" as Jack Vance would say.
BTW a vindictive DM would have ruled and "let" the magic user cast that prismatic wall in such a manner that it was say 3ft above the floor in the room the PC's were so more/all the PC's bisected the prismatic wall.
A brave choice was made without fully considering the risks, I think the DM was within the scope of what was reasonable based on the info we've seen so far.
As DM I like to use telekenisis to throw PC's into Prismatic Walls and Prismatic Spheres and have on several occassions in 1st ed AD&D.
As for it being an edition thing I don't think so. Still seems like their is far more save or die and instant death in 1st ed AD&D, even at the lower levels. Of course any version can be deadly but 3e/3.5e with all the excessive worry about CR etc seem to be less prone to result in deadly encounters for the most party unless the player contributes.