Beholders kind of sucked, though we did take on an adventure called 'City of the Beholders' in which we killed MANY of them (but it did require our most extreme 'pull out all stops' play). I recall the final desperation ploy was when my character unleashed a Wand of Wonder into the mouth of one, and grass grew. Don't recall what got us out of that, but yes Beholders are dangerous. Still, that was only after we killed like 20 other ones, and then we teleported out. We never did make it to the end of that adventure though, it was pretty much impossible... (or DM was very evil).Only a few things would make a wizard die fast.
Mind Flayers (as psionic was not magic...)
A high level fighter
A cleric with a nice dispel magic (no more protections)
A lich or Archmage with spell reflection or Mordenkainen's disjunction.
A group of adventurers
Demons and devils in general are pretty problematic, and I'd class Rakshasa with them, though a little too weak to likely deal with Questioner of All Things. lol.
We pretty much ignored psionics, but I agree they are stupid nasty, unless you have them yourself, in which case they are stupid ridiculous powerful.
Dispel magic wouldn't do anything to permanent items. Yes, it would cancel Stone Skin. Flying I got from a pair of wings (yes, do adventure in Gamma World!). Things like Invisibility from a ring are ambiguous, it is a permanent non-charged item, but you COULD treat this as a spell-like effect since it has to be explicitly invoked. Anyway, I'm still flying, displaced, maybe invisible, and with an AC of -2 IIRC. You won't like the response! Heck, you're very lucky you got a shot off, few enemies did.
Obviously opposition spell casters are the main worry. We were very systematic in dealing with them.
Meh, not really.And...
The Crazed Kobold Corps. (Be very wary of them!)
AD&D is a very ambiguously worded set of rules which gives the DM virtually unlimited rope to use to 'kill people', so I wouldn't ever engage in an adventure on the basis that the DM was out to prove that he could kill the 'unkillable'. OTOH, under the conditions of what I would consider the 'Gygaxian Contract' of play, the challenge is set ahead of time, and only specifically designed against a given set of PCs if that makes narrative sense (which probably means they know what they are in for). Under those conditions our approach to adventuring as high level PCs, and given our equipment and whatnot, made us very resilient. We did find a few challenges which thwarted our efforts (very extreme ones, like the above mentioned CITY FULL OF BEHOLDERS) but nothing ever killed the core characters (we lost many lower level PCs though).Over the years, I have killed my share of "unkillable" wizards and groups. Sometimes with NPCs and monsters that were way lower than the "supposedly" invicible characters.
I looked at it like this, Questioner of All Things, 14th level wizard. He's a 180 IQ super genius. He doesn't make mistakes, doesn't take chances, doesn't fall for ANYTHING. So I would sit down after learning what we were thinking of doing, and come up with a systematic plan, budget, contingencies, etc. Then we would send in scouting parties, build advanced bases, cache materials, or take other similar actions, including mapping out locations we could teleport into or out of, etc. THOROUGH plans, like if you were real-world military grade types of plans. If we thought there was any special equipment, spell, item, etc. that would be valuable in the situation we acquired that. Etc.
Yes, there is the unexpected, but quite frankly if we were presented with a situation that sounded like "go somewhere unknown, without some logistically sensible backup and line of retreat" then we simply passed on it, or waited and prepared, whatever the cost of doing that was. When you approach adventuring like that, as a high-level caster, it is EXTREMELY hard to be totally caught out and outright defeated.
Of course, the bad guys were equally nasty! There were plenty of times they pulled stuff on us and we just said "OK, lets just take our losses." Even to the degree of abandoning holds and whatnot if the situation looked like some evil spellcaster you didn't have intel on was coming for you.
Again, obviously the DM could, in principle, just rule all your preparations insufficient, not carried out thoroughly, undone by some henchman, etc. to the point where you were utterly defenseless. Our DM was diabolical, but he wasn't that much of a jerk. Now and then he'd wipe the table with us, but we would get away, regroup, acquire more stuff and get stronger, and come back.
Invisibility is just one 'layer' in your defense. The problem with fighters is, they lack all those layers. Yeah, they've probably, realistically, got good AC and saves, and they could have other things like wizards could have, to an extent. They just needed to find all of it. I can show you very strong fighter sheets from the day, but defeating them is much easier. Defeating a caster is a lot harder, especially if you play 'to the hilt'.And magic items were there exactly to give the fighter a "fighting" chance to shine in the group. Magic items were not common, but could be found in enough amount to give a 12th level figther an average AC of -2 for 3 attacks and 1 for the rest of the attacks (Plate +2 and Shield +2 would be good). Then add a ring +2 and the AC is now -6/-1. Nah... the fighter was the shmook you seem to think he was. And with a flying potion he could reach your friendly wizard. And with a mere 11 of intelligence, your lowly 15th fighter had a 55% chance to know, yep, know the exact location of your invisible flying wizard. The higher the HD/Level, The higher the chance. Even at 12th level he'll have 25%. And once spotted/attacked, he only suffers a -4 to hit. And the rule do say that once attacked, the creature can always defend/attack the invisible creature albeit with a penalty to hit. Invisibility was not the perfect thing most people make it to be. (for the invisibility, DMG1e p.60). Many DM ignored this rule and that table.