First of all, I'm disapointed that this isn't Isaac VS
Solaire of Astora, the joke pretty much writes itself.
Secondly...I'm not sure how Isaac can win, but I'm pretty sure the Ashen One can't win either. Isaac seems to have a much larger arsenal to work with, more elements and powers, much better range, differant types of healing, stat manips, forcefields, a lot of resistances to elemental manipulations, and a collection of various statuses and conditions he can deal out like Power Nulification, Immobilization, Sleep, Curses, 1-Hit-Kills, ect, ect, ect.
But while he is able to resist, out range, or heal off pretty much everything the Ashen One can do, the Ashen One will always just get back up and be ressurrected again. I don't think Isaac has anything in his arsenal that can make the Ashen One go away long enough or be permanantly destroyed for good.
I don't think the Ashen one can win, because looking through thier scaling chain I can't find an AP, Isaac outranges, and also has a lot more to hit with while resisting so many differant types of attacks. They might be able to pull sneak attacks with silencing and invisibilty, but without being able to go for a one-shot, we're back to square one where Isaac seems to take control of the fight will all his powers, Djinn, and summons and heals off anything the Ashen one managed to do.
Isaac's only option, I think, is to use the Gale Djinn to make the Ashen One flee from the battle, but I'm not sure that'll count as a BFR win because there's like, nothing stopping him from comming back. Maybe Iris throwing him into space might count as a BFR? Incap via freezing or sleep or immobilization maybe? If Isaac wins, it's going to have to be a roundabout way
Otherwise, Inconclusive, I don't think the Ashen can win (MAYBE wait for Isaac to die of old age, but thanks to the Golden Sun he's looking really damn good for his late 40's, he might have longevity), but Isaac can't stop the ressurection cycle.
Also, while it's super unlikely to ever come up, the Ashen might be able to take control of the Dullahan should Isaac ever try busting out Tomegathericon. That won't change much, since Isaac should be able to kill the Dullahan at his peak, but that's a funny thing to think about.