While a small enough, fireproof, super strong Demon could sneak up to said Dragon, climb up to its mouth and make her way through its body until she reaches something vital to destroy. Like its heart.

But honestly, I dislike the dragon method.

That aside, if this dragon is so massive and powerful that is could literally demolish towns just by landing on them, why exactly did humans win this 'war'? For that matter, what do any of the player characters matter if they could all get erased by this behemoth who could do a much better job of liberating the world alone than if they all teamed up without him?