In the Call of Cthulhu d20 book, there are stats for the gods of the Lovecraft mythos. Azathoth is the most powerful IIRC, and while I've seen some extremely powerful characters on these boards, I don't know how many of them could kill a god, barring Pun Pun of course.
Here are his stats.
Azathoth
Colossal Outsider (Greater God)
Domains: Evil, Chaos, Madness, Death
Hit Dice: 72d8+2091 (2,666 hp)
Initiative: +5 (Dex, Improved Initiative)
Speed: 140 ft., fly 360 ft.
AC: 63 (-8 size, +1 Dex, +40 natural, +20 divine)
Attacks: 6 slam +102 melee
Damage: Slam 4d6+17 (improved critical) + Int drain
Face/Reach: 40 ft. by 40 ft./25 ft.
Special Attacks: Int drain, wail of madness, alter reality, divine celerity, squamous blast
Special Qualities: Divine qualities, remote sensing 20, damage reduction 55/+4, SR 52, darkvision 60 ft., divine fast healing 200, fire resistance 40, cold resistance 40, sonic immunity
Saves: Fort +89, Ref +61, Will +61
Abilities: Str 45, Dex 13, Con 69, Int 3, Wis 12, Cha 34
Skills: Concentration +104, Cthulhu Mythos +71, Knowledge (planes) +71, Hide -15, Listen +66, Spot +3
Feats: Alertness, Cleave, Dodge, Endurance, Great Cleave, Great Fortitude, Improved Bull Rush, Improved Critical (slam), Improved Initiative, Iron Will, Lightning Reflexes , Mobility, Power Attack, Spring Attack, Sunder, Toughness, Weapon Focus (slam)
Climate: Any
Organization: Unique
Challenge Rating: 50
Treasure: x5 standard
Alignment: Chaotic Evil
Advancement: None
So, my challenge is, find a character that can kill him. He actually spawns with minions, but for the purposes of this, we'll have it be a 1v1.