First off, your DM stole my idea! /ranting fist
Secondly, when I played H4 way back in the day we had a bizarre group of 1st ed barbarian, solar (me), wizard, paladin, and cleric. When we encountered Orcus we thought we would tear him a new one. Boy were we wrong! It nearly ended up being a TPK, and would have been if I hadn't run away (technically I didn't run away, I ran "to go get help" but was unable to find any).
To answer your question, I'm not sure any one group is better than another except that you can most likely expect to fight lots of
a) undead
b) demons
Good luck!
P.S. My current campaign I am running intends to end with a romp through Orcus' layer of the Abyss in an attempt to wrest his wand from him. We're several years of play time from that happening, though.
Cleric with the Sun and Glory domains, THF paladin, archery ranger with appropriate favored enemy, TWF rogue with Holy weapons, Diviner wizard. Given the heavy focus on demons and undead, building a party optimized to destroy those enemy types is an effective option. And a Diviner wizard will always go first in initiative, meaning that with Time Stop and quickened spells, he can front load a whole crapton of damage at the very beginning of a fight.
Well, iirc, at the time we played H4 the module gave Orcus the powers of a deity (perhaps even a greater deity?) when in his home plane, so you can forget about diviners always going first. *grin* In the version we played, Orcus knew all and could just about do all.
Then again, I think we missed the mark when we decided we would "pry the wand from his cold, dead hands." Attacking a greater power in their home plane when they are prepped and ready for you was folly, and we paid the price for it.
My solar made it back to the prime material plane and attempted to gate Fraz urb'lu (sp?...the demon lord of deceit) thinking he might send Orcus instead. Nope, he came himself and told me to bugger off. /sigh
I'll leave build suggestions to others. My advice would be to remain vigilant, don't get cocky, and look at what your true goals are. Good luck!
For the most part the builds can fit standard builds of the class type.
Some more specifics might be: Give the paladin Oath Against Fiends and Oath of Vengeance. Oath Against Fiends carries a handy aura that causes teleportation and summons to be tricky, and Oath of Vengeance allows a paladin to grant his allies the damage bonus of his Smite Evil (which is a whopping +40 damage at level 20 against evil outsiders and undead). A Holy Avenger or a Sun Blade are decent weapon choices, especially if your DM allows you to upgrade them and/or have them come in alternate weapon forms (like a Falchion). Otherwise, you can go with things like Holy + Bane on your weapons, always remembering to have Bless Weapon active (making him automatically confirm crits against evil enemies. Wielding a falchion with Improved Critical will net a 15-20 crit range, and Smite damage is multiplied on a crit) The rest of the gear should be pretty standard (Ring of Protection, Belt of Strength, Amulet of Natural Armor, Cloak of Resistance, etc).
The cleric should have a decent Charisma for better channel energy, as the Sun and Glory domains increase the damage and DC of his channel against undead, while reducing their chance to resist it. You can also make sure he has decent physical combat ability, to be able to wade into melee and take advantage of the paladins shared smite. Righteous Might, Divine Favor, etc are good self-buffs to make him a melee beast in his own right. Depending on the god (or a possible fighter splash) he'd have access to decent martial weapons and heavy armor. And of course the standard array of clerical buffs and debuffs to help the party.
The Archery ranger should stay pure ranger, and stack his Favored Enemies to either make Demons +10/+10 by level 20, or split it with undead. Alternately, he could take the Guide archetype and just choose specific enemies (which would stack with Quarry), granting him abilities to force rerolls, among other things. A level 20 ranger would be firing 6 shots a round (7 with Haste). With Deadly Aim, favored enemy, quarry, and Smite, each one of those shots would have +62 static damage, a massive +hit, and automatically confirming criticals. And that's not even counting magic items or strength from a composite bow. Plus, you'd have a decent animal companion.
The rogue would be a standard TWF build, probably with kukris for the improved crit range to further take advantage of the Smite bonus. You can give him teamwork feats like Outflank and Paired Opportunists. Butterfly's Sting is another potent one, as you could give one of your crits to someone like the Paladin, who is likely to hit harder. If you go the Teamwork route, be sure that the Paladin and/or the Rangers animal companion takes them, too, for some potent synergy.
Having a Diviner wizard is potent at the beginning of a fight to front-load damage spells with Time Stop, and possibly chain Wishes in that time to debuff Orcus, reducing his offense and defense. Other than that, standard buffs like Haste and Displacement are useful to provide to the party.
Human Cleric of with Sun and Glory domains. All favored class bonus going to +1 caster level vs. Outsiders. Take Sacred conduit to boost the your channel energy.
Half Orc Paladin using a two handed weapon.
Dwarf Spirit Ranger going the switch hitter router. Favored Enemies Evil Outsiders, Undead, with the Abyss as a favored terrain all favored class bonus to HP.
Elf Wizard diviner with all favored class bonus going to increase the number of Diviners Fortune
Half Elf Arcane trickster (3rd level Wizard, 3rd level Rogue, 9th level Arcane Trickster). Useful for extra spell casting and traps. Take Magical Knack trait to boost spells.
Human Bard with all favored class bonus going to extra spells. Focus on Archery instead of melee.
If you have a sixth member I would go with a Human Oracle of Life with all favored class bonus going to spells.
All but two of the characters are healers of some sort. This should allow you to recover from anything but death in no time. You will also have three characters able to channel energy to help with the undead.
If the Cleric, Wizard and Arcane trickster are summoning creatures you will have an army at your disposal. The Bard will boost everyone combat abilities to make it even worse.
All the characters will have decent save including the martial characters. Actually they will probably have the best saves of any one. This will go a long way to keeping you alive.
If you do have the Oracle of life that is going to free the cleric up to buff the hell out of the rest of the party.
Do you have access to Tome of 9 Swords? if so, the Diamond Mind save feats are going to be very handy.
Soulforge Armor. You need permanent Death Ward. And immunity to poison..a level of chevalier?
A reusable way to restore ability damage that gets through.
Fort saves to the moon. Immunity or extreme resistance to charms and possession.
==Aelryinth