How is the amount of AP needed to gain a sphere level determined?

What do you mean? As you progress in the game?

I think it might be 5xy + 5 < 22000 or something like that, when x is the total amount of levels gained and y is the total amount of bosses and tough enemies killed, as long as it doesn't exceed 22000.

Thanks for confusing me now.... ;-)

......Or I suppose it could just go up by 5 AP every time you gain another level as long as it doesn't go over 22,000 AP. :uh?: