And I'm also saying that I'm afraid Neji might not want us killing God!
Here's a basic outline:
You should be able to fight out of free will, like a optional boss, because he's so strong (think of Yiazmat in final fantasy alternate)
Something like.....
1000% health
300% attack
300% defense
Ability to call up ghost minions that are like regular strength forest creatures
Able to face him at level 15 in some area, say the meditating area in the gardens or the temple of shadow and light
Maybe you might have to have an item, like after you beat Orochimaru you have the chance to grab a certain item and it keeps you from the whole reincarnation event. You should be able to take the item to the certain place and summon shinigame, big dramatic talk that reveals he saved you and that you shuld be in eternal service to him. When I think of the battle I kind of think of the battle with the emperor in FFII soul of rebirth. If you lose you go straight to hell (ummm.... shades) with no torments and no karma. You have 3 more chances to beat him and if you don't you go straight to your next oro kill phase but with the negative buff that decreases your turns by 5 and your attack and defense by 10 (attack and defense last 70 rounds), whatever you want to call the buffs I don't care. If you win you should start out with 100 karma and the 500 karma weapon and armor as soon as you get resurrected.
I personally think the negative rewards should be larger but whatever, tell me if you think the reward should be bigger or smaller, at first I was thinking no reward at all. It's just a rough outline so feel free to criticize.