I don't see how an Epic Spell with a DC 85* would be impossible in light of spells like Famlicide and Cloister. Even Xykon's Superb Dispelling has a DC of 69 (if we remove the mitigating damage factor).
Furthermore, there are mitigating factors to the spell by increasing the casting time and adding an XP burn. This will reduce the Final Spellcraft DC. Considering the nature of this type spell, it's not unreasonable to give it a casting time of (for example) 65 days and burn 5,000 xp.
* While, I have not crunched the final DC but I imagine this sort of spell would have an even higher DC.