
So basically, in my opinion, a character is too skilled when there is absolutely zero plausible room for another character to exist in their story. Just as the Warrior of Light does not truly have party members or characters that assist them with battles, if your character is so skilled they are not in need of anyone else ever in any way, then that is when they are too skilled, TO ME.
EDIT: Id also like to add that the older a character gets, naturally this line can get harder to maintain as naturally a character in a story might try to cover their weaknesses and bases. So I think having an established character "Existence" is important too, since your character having all of these developments off screen tends to make it less....realistic(?) for many people.
Beware, Rpers be a fickle and critical bunch at times, yar.