Ideas/advice for a level 20 mythic one-shot?

[slachance6]

I've organized a short adventure (several hours) with level 20, mythic tier 10 PCs. The characters are a paladin, rogue and sorcerer (pregens that I made), as well as a hunter and a gunslinger that two of my players created. Most of my players have never played any mythic game, and none have played any game at this level. None of them are huge powergamers either.
I was hoping to have some kind of player-vs-player element in the game. It doesn't have to be a pure combat, but I want the players to have different motivations and goals.
Also, I'm still thinking of a plot device that will hold the characters together while still giving them conflicting goals. A macguffin seems too cliched and one-dimensional.
Third, what are some good monsters or encounters to include at this power level? I'm pretty sure any one of these characters could probably solo the Tarrasque.
Any other advice for this ridiculously over-the-top adventure?

[Thealtruistorc]

Stats exist for most of the Demon Lords. Have the players blitz the Abyss killing as many apocalyptic entities of utter desecration as they can (meanwhile, the demons will keep a tally of how many heroes they kill in the process).

Alternatively, bring back Tar-Baphom (also statted) and send his undead army after the heroes from all sides. The inverse could be utilized if you want to try and conquer Geb (using Arazni in place of TB).

My last idea is just picking the final bosses from a whole bunch of adventure paths and have them all team up to fight the heroes. I envision a massive join attack involving Karzoug, Unity, Xin, Elvanna, and a bunch of other superbaddies would be something not only fun to think up but also fun to create.

As for advice: have a lot of enemies and a lot more backup plans. Mythic PCs can do stupid, stupid things, and this goes double for level 20 ones. You should be playing just as rough as they do, exploiting dirty tactics and doing everything in your power to get the upper hand. If that means using high-level spells in cheap ways, do it. If it means infinite loops, do it. If it means crazy homebrew artifacts that force the PCs to change tactics, do it. They are begging for a challenge when they ask for this one-shot, so go nuts and pull absolutely no punches.

[Barstro]

I recently started a Mythic character and I already think he can do too much.

My initial thought was (since I have trouble finding a reason to have much roleplaying or character development in a one-shot) to have a dungeon crawl where the mooks are about level 15, and things escalate the closer you get to the end goal. I suggest this only to force some sort of resource management on your players otherwise I expect it will be nova-rinse-repeat.

But then the concern about resting entered my mind. Without level 20 and without any Mythic, PCs still have way too many options to flee to safety and rest.

So, my actual advise is to figure out a good and logical reason the PCs cannot hit their panic button or otherwise continue to rest and attack at full strength all the time; a major time limit, some sort of negative field that hampers healing, a nightmare realm that interferes with spell slot recovery, etc.