Well, in actuality, the "whittling down" occurs in about 1-3 rounds, it's just that the tactics involved, and the die rolling, etc. make it take a long time, chronologically speaking. A 3-round fight might take an hour to roll out.
And sometimes, yes, it is "live or die" / "save vs. death," although the players got smart about that a long time ago, and started having the wizard and the cleric produce ward items for the party.
And sometimes, yes, the cleric uses 11th level heightened Miracle to duplicate Time Stop, and then the party (all of whom have Spell Stowaway: Time Stop) unloads a veritable "blast" of spells which occur at the end of the Time Stop spell, usually obliterating most of their foes.
But I try very hard to present them with foes that require everything at once from them---sound tactics, judicious use of spells, proper preparation, and good, old-fashioned ingenuity.
But come to think of it, I am beginning to see that the NPCs they face are the ones that are particularly deadly, while the "run-of-the-mill" Paragon monsters create combats which are less lethal, and more akin to a long day plowing a field, LOL
I have looked on the internet for "epic monsters" and I have found only one that would be a proper challenge; unfortunately, he would EAT the epic party, because he is TOO powerful in my opinion. I am referring to a particularly GRUESOME (yet, fair and balanced) NPC created several years ago by Blackdirge, by the name of Yaghur Hod. He's the ultimate Slaad lord, and his combat prowess is SCARY.
The scariest being they have faced thusfar was the lieutenant of Vecna, an Arch Lich by the name of Deverast The Tainted. He was Fighter 13 / Wizard 36 / Divine Rank 2 and they managed (barely) to imprison him within a soul-draining staff that they got from another lich known as Algothmar The Dark. This led, of course, to emnity with Vecna, which opened up a whole new can of worms. [Ironically, Algothmar was once the lieutenant of Vecna, but he fell out of favor, and was replaced by Devarast. The party had previously defeated Algothmar during a separate, un-related encounter, and when they obtained his staff, they unknowingly obtained just about the only thing capable of defeating his replacement, lol.

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As to "absolute immunity," they don't have that, but they have HIGH resistances, and SOME immunities, combined with ward items and epic buff spells, which make them pretty hardy.
Basically, I follow one simple rule: I never give an NPC a power or ability that the PCs can't earn or otherwise have. I also make absolutely sure that NPCs plan things out as well as the players do. Foes that they randomly encounter typically die, and very quickly; these types of foes just aren't prepared to face the onslaught that these guys are capable of. But foes who know them, and what they can do, never attack them without EXTREME preparation. Thus, they live their lives on the edge, always looking for ways to improve their buffs and protections. And maybe that's part of the problem; I have scared them so bad in the past, that they have begun to focus more on defense than offense.
By the way, as to rulers of the lower planes, I have taken measures to improve them. Graz'zt, for instance, is an advanced (to 39 HD) Balor, 18 Rogue / 10 Assassin / 20 Wizard / Divine Rank 0. His acid sword is a MAJOR artifact. He's no pushover. And he has hordes of demons at his command.
(I hope my players never see that last bit of info, lol

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