I'm not sure I see the problem.
A character who is a demigod, can - if he uses two dailies (a level 25 and a level 26) - have an excellent chance of taking out any single opponent. Assuming the opponent's allies don't have a way of stopping him.
So what's the problem?
That's pretty much what I would expect a demigod to be able to do.
What makes Orcus so special? If you are going to stat him and put him in the book as a monster, then implicit in that approach is that he can and should be killed.
He's ONLY level 33 and thus - like any other level 33 monster - ought to be in reach of a party of level 26 plus. Its not about that one encounter, its about that encounter, plus the surrounding encounters, plus whatever the goals of the adventure are.
Its not an MMO(RP)G built around raids to kill boss mobs when they respawn, its a roleplaying game. You don't have to worry about whether this build or that build has an unfair advantage over some other build. You just have to worry about whether your party is challenged by your adventures.
Carl