See, there is a very easy way to dodge this issue, and it is simply not give the villain a power or a feat that makes it feel like they completely overpower the heroes, nobody can say "Wait why didn't the Big Bad just kill the heroes with his mind?" if the Big Bad is never shown or is suggested to have the ability to kill people with his mind, and again, the issue isn't just Fandaniel, Ascians in general are depicted as being completely untouchable until the moment we are supposed to fight them, at which point their ability to impecably teleport away from attacks suddenly vanishes, this isn't even an EW issue, it's a game-wide issue.
I also want to make clear that, at least to me, this is by no means story-ruining, I am used to stuff like this for the reasons you mentioned, but I also feel that a story is made better when the writers put in the effort to avoid tripping that flag.