User blog comment:Wrath022/Chapter 524: Black Future, Review/@comment-5303512-20170227145147/@comment-110.55.3.104-20170228005315

I think what is meant by August 'knowing all magic' is that he has a profound ('scientific' or 'mystical') understanding of the foundations of magic as well as the conditions involved in (almost) every spell. This doesn't mean that he can immediately use all spells (if he himself doesn't fulfill the prerequisites for some of them) but he is in a position to thoroughly analyze each spell according to its underlying principles, and counter them accordingly if it is within his power to do so.

With that said, sudden character silliness aside (much to our disappointment), a plausible way for the 'king of magic' to be defeated in battle would be if he is overwhelmed by magic power so great that he can't handle it even if he can understand it, or if for some reason someone constrains or bypasses August's magic ability and proceeds to take him out by physical means.

I can only wish that before the imminent downfall he would show even just five to ten more different spells including the power of feelings (he is a loyalist through and through after all)