Zanz is the developer. So, obviously, in the game he is God. The True God. In relation to your story, he could be the original being in the universe, or what's left of him after he split.
Everything is what is left of him after he split though, unless you are saying that the split was unsuccessful and a large part of him did not leave at all. In which case it would not be one being the mind would still be fragmented within Zanz there would just be one being holding them all together.
It doesn't have to be his avatar; it can be himself.
Better.
All elementals descend from Zanzarino; they all used to be a part of him. So any elemental that emerges triumphant over Zanzarino will get to re-merge with him and gain his power, thus becoming the new God.
A loop you say? An elemental re-merging with him makes sense but the way you say "gets to" makes it sound too much like a prize or a reward. Remember the Elementals originally split precisely to escape from being part of this being. Why would they be so eager to re-join after defeating him? A re-merge would have to be against their will and his too.
He is the final boss because he is the True God, and the strongest AI you can ever hope to face. In my mind he's someone with 500 HP, infinite quanta, and infinite cards. Of course, when you become strong enough to face Zanzarino and not get annihilated to pure quanta just by being in his presence, that makes you already stronger than most False Gods. In my mind, you gain the power of a FG when you fight Zanzarino.
This is fine of course but is kind of seperate from the story.
You said the original being is lonely. Maybe he's tired of everything, and he wants to pass his mantle to another being (one of his children). But by themselves the elementals can never surpass him, so he gives pieces of the future to elementals to see who is worthy.
Wishing the elementals to surpass him makes sense. But what do you mean by "gives pieces of the future to see who is worthy"?
I thought you'd be more imaginative... Wouldn't all people assume that Zanz is the uber creator god? 
This was your idea not mine. I want the fundamentals to come from you and not have large parts of it written for you. I would rather challenge and question you so you must explain it more thoroughly then simply fill in the gaps myself and end up going against your idea in the process. The problem with somebody else adding things too early in the process is that it can ruin what you had in your head but haven't fully translated intow rods yet.
And as the great one split himself, and his work was complete, he heard a laughter from behind him. He had never been alone, there had always been one, watching. This man, a happy looking fellow, had malice hidden deep within his bones. He spread the children of the great one to great warlords, who used them to wage war on each other for sport and money. He helped some become neigh gods, and helped others take these gods down. He was all knowing, had infinite amounts of power, and was content to watch the world rip itself apart. And so, as the great one's lands descended into war, the oracle just laughed his hearty laugh, and waited silently once more.
I think it's okay, but critique is welcome.
It's not bad at all.
I would suggest two things.
First should come a reason for why or indeed how the Oracle kept himself hidden from the first being.
Second I would remove the malice clause. Is it Malice? It puts me in mind of a child watching ants fight, he enjoys the turmoil of something smaller then himself and to do so is not noble but is it really evil? It is more thoughtless and dismissive, we often do not care about things far smaller then ourselves. We step on ants, and spray flies with chemicals and destroy spiders webs. Would Gods not view smaller beings with the same indifference?
The Oracle finds great amusement and perhaps even draws his own strength from the conflict and turmoil, using it to feed himself and this may be why he would give power to the elementals. (and also maybe why he did not announce his presence to the first being in order to grow from it's problems)
I haven't added any more to the original post in a good while. Been kinda busy, bear with me I haven;t abandoned it or anyway. It'll be finished before too long.
