If there's one thing Whitey likes, it's challenging the PCs.
Challenging them until they collapse in piles of gory ruin.
To facilitate this, there's basically two approaches to take with even 'mid' level characters.
1 - Nullify their ability to do stuff. Rods of
cancellation, level or ability drain,
disjunction, and on and on. The party described here looks well stocked between casters and brawlers (just an NPC cleric, though) so this would require a diversifed approach. They won't much like this - but when have they ever liked what the hostiles have tried to do to them?
2 - Require they use all their stuff. By this level, there will likely be one or more hostile organizations looking to spoil their day. Every day. Have them face (picking some random examples here) rampaging troll/giantish hordes to the south, diabolical court intrigues in the west, inexplicable geologic disasters to the north, and superelite hit squads from the east. All for them to deal with personally - people expect that of their heroes. All in one busy day.
Viewing this more abstractly, instead of placing pressure on spell allotments or riches or even HP, you could aim for time. Maybe the trolls don't cause any significant wounds to the PCs, but they use up daily slots, spread the party thin and disrupt their synergy, and cause collateral damage that the PCs are expected to remedy. Don't forget that while they're going toe to really big toes with the giant chieftan, someone up north is now under an avalanche wondering where the heroes are. They'll need to make some tough choices, ones with consequences - great power comes with great responsibility, and great expectations.
A CR 20 lich is not going to tax a group of 19th-20th level players unless they're unprepared for him and/or have had their resources already taxed. A full-powered party at this level will be quite powerful, and will easily be able to handle a CR 23+ encounter, if they're prepared or have enough resources to meet the challenge.
No foolin'. The whole CR system starts to crumble even worse as levels go higher. The amount of nasty coming in at one target far outstrips what that target can return. By level 15, Whitey would say add +3 to the level/CR of any hostile you intend for the whole party to mob, and +5 if the party has prepped with suitable gear/spells and can fight on their terms.
Try the following brute squad, and see how the PCs fare.
Nalfeshnee [MM1] bumped to 15HD and given Improved Sunder.
A Wiz13 (Illus) to baffle the boomer. Creates the illusion that all sorts of horrors are on the loose.
Invisibile StalkerRog 5. Classic RB combination.
Roper. Haven't used one in a while, and it'd be a good guardian type for your undead's crypts. It also has good synergy with the hostiles mentioned above.
This encounter would give a good estimation of their abilities. If it works, Whitey will even help you write letters to the party's next of kin.
