And as long as we're talking about it, Therkla left halfway through the fight, was probably significantly lower level than the PCs, and wasn't in a position to sudden strike. Hinjo is 12th level or so, but was taking some hurt by then, and his dog was out of the picture. Lien is even lower level, and was having trouble beating the devil's DR. Elan isn't terribly good at melee even at the best of times, and probably wasn't packing a silver rapier. Durkon is a healer, not CoDzilla. So it wasn't a pack of 15th level characters going after a CR 20. Additionally, the reason the CR system seems a bit low is because most PCs optimize their characters significantly more than the Gimbles and Jozans the system assumes. THis isn't true with the Order. So really, the best option against a single overwhelmingly difficult opponent IS to lower its saves and throw save-or-dies.
As for the AC battle, consider what would have happened if either side hadn't had their high level characters. Without Redcloaks summons, the walls would have held much longer. V would have been free to blast the hobbos all day long without the Death Knight, and his enchanced soldiers could have kept killing. Xykon wouldn't have wiped out the occupants of the keep.
On the other hand, Xykon has DR enough so that the common soldiers can't touch him. He could have bombarded the soldiers from his dragon (that they can't hurt with arrows either) until he runs out of spells, then land and make touch attacks until he re-dies of boredom. Without the hoards of ghosts in the throne room, he and Redcloak would have been virtually unopposed.