Like the other GL fight all I can do is throw out what I know of Jeroen and see what you have to say about it.
He's got telepathic attacks which you said were effective against normal GLs so that's something. He can move people and things to different dimensions (like the one he sent Apollo to where speed turns into noise or something like that). His signature move is turning stuff into other stuff (people into butterflies, spaceships into cupcakes, exploding gore into rain, etc) but I don't know how that would affect Ion. Is he immune to being transformed? Time travel and control is an option (though I think Ion can do that too, right?). He could attack him every day as he grows up. And for defense, he can make himself indestructible or just unkillable. I'm basing that not on something he's done, but on another Doctor doing it and him having all the powers and experience of every other Doctor.
They both have obscene amounts of power and it works the same way for each of them. They picture it in their mind and it happens. I'm thinking Ion only because he doesn't have the issues that Jeroen does. If they're equal in power (I don't know, but I think they are) then that's reason enough for him to win. If the things I mentioned above work though, then headcase or not, Jeroen can win.