Note that it doesn't really matter. When you sit down and create a high-level character whole cloth, whoever is playing it won't really have a good handle on that character's abilities anyways.
It's pretty likely that it'll be oriented around a few killer abilities without a lot of depth, and the game itself won't be like a campaign, more like a one-off like "Hulk Vs."
If 3.5 material is allowed there was a metamagic feat called persistant spell, it increased the duration of any non-instantanious spell to 24 hours.
Build:
Wizard (Diviner) 20
Sorcerer 20
Ultimate magus (3.5 prestige class) 10
Initaite of the sevenfold veil 7
It doesn't really matter 33
You can use the ultimate magus to apply the persistant spell to Time stop, Win initiative due to diviner, use one of the several veils to deal a fairly small amount of damage, then cast 24 hour time stop again, rest up, repeat about 1 minute before your time stop ends. All you need to be able to do is 1 point of damage/day and you auto-win.
I wasn't kidding when I said a CR 90 is 4 to 8 billion tarrasques. . . how do these 90 level character builds handle between 4 to 8 billion CR 25s?
1 CR 25 = tarrasque
2 CR 25 = 1 CR 27
2 CR 27 = 4 CR 25 = 1 CR 29
2 CR 29 = 4 CR 27 = 8 CR 25 = 1 CR 31
2 CR 31 = 4 CR 29 = 8 CR 27 = 16 CR 25 = 1 CR 33
That's an exponential increase. The number of CR between 89 and 25 is 64. There are 32 2 CR pumps between 25 and 89 (25->27->29->31). The number of base creatures (CR 25) at CR 89 is 2^32 = 4294967296 of the base CR 25. The amount at 90 is somewhere between CR 89 and CR 91 (which is 2^33 or 8589934592).
So I'm not kidding when I say a CR 90 is between 4 and 8 billion tarrasques. If your build can't defeat that many tarrasques, it's not strong enough.
If I remember correctly due a couple of the following things. You can usually pull this off by level 20 so the rest doesn't really matter.
There is a spell that lets you auto-crit on your next attack
There is a spell that lets you auto-confirm a crit
Take a few luck feats to make sure you auto hit in case you get dispelled
Buy vorpal sword
Auto-vorpal anything
End game.