Its kinda like when you have a character hitting 50 times against an enemy but doing less damage than the guy who hits once. The reason is that the enemy has a flat damage reduction going on before you actually hurt it. This ends up making it impossible to kill with attacks below a certain level but easily killed by one big attack if its health pool is small.
He's running into a mechanic of demons which might actually explain whats wrong with this worlds magic. The reason it functions this way may just be because demons were the most prominent threat and magic was created with destroying them in mind. It could also simply be because he is an outsider and everyone from this world was actually blessed by default. The hero would of course be blessed too but since he didn't go along that path he missed out.