Champion is indeed a problematic case in Elemental Game. First, if they are too strong, they can become a one man army, then if they are weak, then they are pointless. Plus, to make the purpose of champion to become a pure magic user will make FE the Heroes Might & Magic clone (so why put the champion as a unit in battle if his / her role in battle is only to cast magic spell? At the end, they will become the same general as HOMM heroes - not a bad idea, but FE should be a unique entity isn't it?).
So, what if we make two phase of champion in the game. The champion as a human and the champion of demi god. At the start of the game, every champion will be considered as human / creature. They will have limited level (Say level 5 or 6 depending on the balance). Then, when they go to a journey and find something important or rare, they have a chance to become a demi god / god themselves. As a god, they can become a one man army without any logic violation (well, they are demi god or god after all) But, to reach the god status, there are a lot of thing that the player must do (and perhap to make some sacrifice to do so), so in the end, a player (or an AI) can reach a demi god status, but sometime not.
A demi god is not a god like creature who will make you an instant win (well, they are, if you let the AI god to reach very high level). The different between demi god and a standard champion is their level limit. So, as long as a demi god still at a low level unit, you can kill them just like you kill another champion. So, when you find that an AI has a demi god among them, you must find and chase (and kill the champion). They won't die, like the other champion, but they will lost their godly status if they are died. In this case, unique equipment like stealth / invisibility will become very important, as you and AI will need them to chase and kill the enemy demi god at far behind the enemy line.
I got this idea from the baldur's gate 2 PC game (forgotten realm ver. 2.0) where they limit your level to 18, and consider that level beyond 18 as a demi god level. Well, maybe this idea is workable. I dunno, I hope it can help.