Honestly...using legitimate characters, no munchins or the like...just about any party would be toast.
Orcus
Huge Outsider (Chaotic, Evil)
Hit Dice: 50d8+750 (1200 hp)
Initiative: +10 (+6 Dex, +4 Improved Initiative)
Speed: 40 ft., fly 60 ft. (average)
AC: 54(-2 size, +6 Dex, +40 natural)
Attacks: Wand of Orcus +66/+61/+56 melee (or slam +66/+61/+56 melee), tail sting +63 melee
Damage: Wand of Orcus 1d8+17 and annihilation; slam 2d6+17, tail sting 2d4+8 and poison
Face/Reach: 10 ft. by 10 ft./10 ft.
Special Attacks: Spell-like abilities, spells, fear aura, poison, summon demons, summon undead
Special Qualities: Damage reduction 45/+7, poison and electricity immunity, resistances (cold, acid, fire 20), darkvision 60 ft., SR 46, telepathy, speak with dead
Saves: Fort +42, Ref +33, Will +39
Abilities: Str 45, Dex 23, Con 40, Int 24, Wis 34, Cha 36
Skills: Bluff +63, Climb +62, Concentration +65, Diplomacy +63, Disguise +63, Escape Artist +56, Gather Information +63, Heal +62, Intimidate +64, Jump +67, Knowledge (arcana) +62, Knowledge (planes) +63, Knowledge (undead) +65, Listen +62, Move Silently +56, Scry +62, Search +62, Sense Motive +62, Spellcraft +62, Spot +62.
Feats: Blind-Fight, Cleave, Combat Casting, Dodge, Great Cleave, Improved Initiative, Multiattack, Mobility, Power Attack, Spring Attack, Sunder, Weapon Focus (slam, Wand of Orcus)
Climate/Terrain: Any land and underground
Organization: Solitary or troupe (Orcus plus 1-2 mariliths, 1-3 glabrezus, and 4-20 zombies or shadows)
Challenge Rating: 36
Treasure: Double standard
Alignment: Always chaotic evil
Advancement: —
Orcus is one of the strongest and most powerful of all demon lords. He fights a never-ending war that spans the many layers of the Abyss with the forces of Graz’zt. Orcus is known as the Prince of the Undead, for it is said in secret that he alone invented the first undead that walked the worlds.
Orcus appears as a grossly fat demon some 15 feet tall. His gray body is covered with goatish hair, and his head is goat-like, although his horns are similar to those of a ram. His great legs are also goat-like, but his arms are human. Vast bat-like wings sprout from his back, and his long snaky tail is tipped with a razor sharp poisonous tip.
Combat
Orcus prefers to melee with his fists. If pressed, he will use his spells and spell-like abilities. He wields his Wand whenever possible. If combat is going against him, he will summon demons and undead to cover his escape.
Spell-Like Abilities: At will—animate dead, blasphemy, charm monster, charm person, deeper darkness, desecrate, detect good, detect law, detect thoughts, dispel magic, fear, greater dispelling, lightning bolt, polymorph self, pyrotechnics, read magic, see invisible, suggestion, symbol (any), telekinesis, teleport without error (self plus 50 pounds of objects only), tongues (self only), unhallow, unholy aura, unholy blight, and wall of fire; 1/day—feeblemind. These abilities are as the spells cast by a 35th-level sorcerer (save DC 23 + spell level).
Spells: Orcus casts arcane spells as a 35th-level sorcerer (save DC 23 + spell level) and divine spells as a 35th-level cleric (save DC 22 + spell level). He has access to the domains of Chaos, Death, Evil, Destruction and War.
Fear Aura (Su): All within 60 feet; Will save (DC 48) or flee in fear for 2d6 rounds. If the save is successful, that creature cannot be affected again by Orcus’ fear aura for one day.
Poison (Ex): Tail sting; initial damage 3d6 temporary Constitution, secondary damage death, Fortitude save (DC 50).
Speak with Dead (Su): Orcus can, at will, speak with dead (as the spell of the name).
Summon Demons (Sp): Three times per day, Orcus can automatically summon one balor, 1d3 nalfeshnees, or 1d4 mariliths.
Summon Undead (Sp): As their prince, Orcus can automatically summon 100 HD of undead per day.
Demon Qualities (Ex): Orcus is immune to poison and electricity; and has cold, fire, and acid resistance 20.
Telepathy (Su): Orcus can communicate telepathically with any creature within 100 feet that has a language.
Wand of Orcus: Mighty Orcus wields a huge black skull-tipped rod that slays any living creature it touches if the target fails a Fortitude save (DC 40). Orcus, can at will, shut this ability off so as to allow his wand to pass into the Material Plane (and usually into the hands of one of his servants). In addition, the Wand has the following powers: 3/day-animate dead, desecrate, deeper darkness, fear, and unhallow; 2/day-blasphemy and unholy aura. Each ability is as the spell cast by a 20th-level sorcerer (save DC 30 + spell level).
Now with demi-gods...it is possible.
If the characters combined have at least half of the hit points and combat potential then maybe...but those characters are world movers and unrealistic.
“The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of the infinity, and it was not meant that we should voyage far.”
― H.P. Lovecraft