Even worse seems to be, that it looks like he is meant be fought in a group. As the hero's group is around the hero's level, when they arrive at the endbattle, it is a close and challenging fight, which can only be won by the power of friendship.
MC won the first time around, because he got a glimpse of the teachings of slime-sensai. Even though the constant healing (exp-leeching) and sadistic antics of his comrades kept his level at bay, his 10 year combat experience (the real one, not the points awarded by the system) and those acquired slime cheat abilities gave him the edge over his own party and made it possible for him to slay them. Demon King was also scaled to MC's high (yet not world breaking) level and was overpowered by that edge.
But that advantage becomes less significant as he levels up. So facing the DK is far more troublesome, if his level skyrockets, than it he would be, if he just faces him with level 15. In former case he would have to battle him for hours in an all out face-off, while in later case, MC would just use his pumped up arms to slam the DK's head through the next wall withing the first few moments.