I made the comment a few chapters back that maybe the reason he levels so slowly was because he's counted as a person for each skill he gained (I counted 17 glowing skills that he was given), meaning he requires 17 times the xp.
Someone else said that stats don't seem to matter, because he was able to kill level 90+ enemies in bulk very easily.
But what if the 17 skills not only made him require 17 times the xp, but also gave him 17 times the stats? And then you have to remember that he has a luck stat that's quite literally double the maximum possible. So any hit will 'luckily' hit the critical spot and 'luckily' do critical damage, etc.
That would explain why he was able to kill things so high above his level. Because the stats sure seem to matter with everyone else. The entire surviving class was having trouble against a single monster just a few floors down, after all.