midnight_v: If you can get past the "walk into Hell" part (assume a convenient portal) and find the demon (assume plot convenience), you still need to contend with the Hellish environment and the other creatures and hazards in the area. How do you do that? With magic and psionics. A "Samurai" who's a Warblade/Wizard/Jade Phoenix Mage could do it, but a plain ol' martial adept couldn't. (With the right items, anyone can do it.)
1.
The Journey I mean like Orpheus, Izanagi, and Inanna if you're familiar. So yeah I don't know if there's a "convient portal" or if you just start walking and will yourself there like any warrior on a spirit journey except this one leads you to the hell of your choosing.
So either is fine, you pick up your "swords" grit you teeth and start walkng into the desert/wilderness and end up in limbo or what have you. If he has to walk to a "place where the walls are weak" etc... well whatever thats fluff.
Point: the Major thing that keeps non-casters out of high level adventure is "you can't get there" that would have to stop. Finding a sustainable fluff is just a part of it, redefining "how magic interacts with non mages" is a part of it to.
2.
Hellish Enviornment sufficently high level non-casters need to be able to "Not care" about things like "ambient temperature" (even in hell) or really eviornmental hazards in general.
You're level 20? Go head swim to the bottom of the ocean. Walk across that volcano... whatever. At certain points non mages need to immunities that they need no care about things like that.
Again: otherwise they can't partake in high level adventure.
A part of it is the lack of mental reinforcement that: Human achievement on earth ends at level 5 or so, our myths don't even go beyond level 10 (if that)
So there really has to be some kind of explanation that at level X: human limits no longer apply. Period.
3.
Other Creatures in the area.The MAIN point of being a 20th level Samurai-Barbarian-Figther etc. . . is the ability to kill things even lots of things.
EVEN ARMIES of things with your shiny sticks. So killing hordes of hellish creatures to find one BBEG is par for course.
It shouldn't be an issue for a last level bad ass to kill. . . 5 vrocks, 4 glabrzeu or 3 mariliths, and they should be able to do it 4 times or more depending on the combination of enemies.
Now there maybe just too many demons in a citidel all at once *shrug* but thats just like having a citidel of high level humans with class levels.
Placing it in hell just drives the point home...
The 20 the level X needs to be able to deal with challenges.
Problem being only 20 level casters can do so with ease and effiency.
Problem 2 being most cr 20 enemies are casters too. ( or puzzle monsters)
Ultimately, what people want is for those character to be able to meet challenges without resorting to hokum and hex.
There should be a melee equivalent to casting solid fog but its really hard to let the melee dude walk around the battlefield making a stun attack vs everyone apparently.