The Highest level PC I've ever seen with my own eyes was 25th (2e) and heard of was 30th (3e). I can't imagine what you do at these levels for challenges. Armies of balors? 1d8+1 wandering tarrasques? Or do you open up the deities and demigods and wipe out one pantheon at a time?
How do you challenge someone that high?
You can't, not in the normal sense. The game mechanics break down.
Consider Waldorf. He appeared in Dragon Magazine, and he claimed to have nuked the world of Oerth (the Greyhawk Setting), rendering it lifeless outside of his own castle, and having imprisoned the dieties in a mine under his castle.
This, obviously, is outside of the normal game mechanics or typical gaming situations.
The responses, from a variety of people who send letters in, all declared hostilities to Waldorf using non-conventional methodology.
The methodology by which Waldorf was finally defeated, ramming a Spelljammer at near light speed into Waldorf's castle, filled with nilbogs, was a very unconventional answer to Waldorf's unconventional situation.
There's another analogy. Consider the nymph. In the old days, in 1E and 2E, a nymph blinded all onlookers who failed their save versus magic (which was a long shot, even then, to make for most PCs, NPCs, and monsters.)
If the nymph was nude or disrobed, a failed save meant death. Instant death.
So imagine the Morgul Host attacking Minas Tirith, and lo and behold, a nymph stands on the walls and disrobes.
Since those orcs all need an 18 to save, and the effect is Line of Sight out to infinity or the visual capabilities of those looking, we are talking about thousands of orcs instantly killed.
So the Morgul Lord, reasonably angered by this, throws Shape Change, 1E.
Remember that spell? Remember what it could do? Any 18th level wizard could have it, and the Morgul Lord could easily have been one, so ... he turns into a balrog, and proceeds to level Minas Tirith single-handedly.
Until, of course, Gandalf uses his own Shapechange to turn into a Solar, and we have a battle worthy of the Last Alliance, at the Gate of Gondor.
So much for any sort of conventional battle.
The same thing happened with the other classes.
With the cleric, Heal/Harm and Resurrection (NO save, MR useless, in 1E, against any of these) Or, maybe, the Super Plague of Spectres?
With the thief, Hide in Shadows 100% and Backstab times multiple weapons (all covered with Instant Death, No Save Poison, or better yet, Morganti Death Magic.)
With the fighter, Haste + Something Else + 2 (or 2+) Vorpal Swords + Girdle of Giant Strength + Gauntlets of Ogre Power + A Ring of Vampiric Regeneration (he does 200 hit points of damage, regenerates 100 hit points from this alone.)
As you said, 1d8 + 1 tarrasques.
Of course, high level characters will presume to *tame* said tarrasques, so they can have them as their *personal mounts*, and if the tarrasques get out of line, just whack them in the head again, and ...
I think the fun was primarily in the Getting There. Once You're There, where to go next?
When you're at the Top of the Mountain, how do you climb higher?