The power scaling in this series is extremely weakly defined or not well thought out all. Like using equivalent exchange on a level 3 thief causes the protag to almost die, but a level 25 Explorer is barely any sweat.
But you may say he had more many abilities by then and he didn't have any by the time he took out the level 3 thief. However, we know that Sherry has been steadily gaining more MP despite having no MP abilities on her class either. There's no way a level 3 thief should nearly kill the protag while the level 25 guy makes maybe a sweat bead drop. Then he is absolutely bodied by poison from the merest knick on a glove from a severed piece of a monster.