CR 15 challenge. Maybe one of the special challenges the specials like to throw in if anyone feel the need for a particularly difficult fight.
I would be a most difficult enemy to fight. I would be invisible with mind blank on. And use Limited wish to cast Spell immunity(invisibility purge).
The battle shall take place in a massive cave. In here various planar rift cause the place to be chaotic and truly dangerous. One place is filled with lava. Another negative energy. And so on.
And for minions he shall have various outsiders bound. Using Greater Planar Binding some pretty high CR creatures can be called. But most likely only a few CR 13-14 will be there. And with a few limitations on what they will be willing to do.
Using his silent rod Seldriss will move around the battlefield, annoying the PC's and killing some. Only a lucky Glitterdust can reveal him. Though if that happen I expect one of the typical Zen archers/ gunslingers/ Fighter archers to take him down. His Stoneskin will not help him versus PCs of this level.
Oh and he will use a variety of cloud spells just to annoy everyone.
(He can see through them himself)
(I expect this to be the annoying 2/3 hours encounter. Maybe one where they wish they were fighting Krune instead :P )