@Linkno i think its more like a system of leveling with each gap making a jump. for example, a fenrir is indeed a legendery S rank boss monster but its not the strongest among the legenderies like the royalty pure blood dragon king and the demon king. so i'm assumming a C dungeon final boss is a S legendery monster that ranks fourth or fifth. which makes a A dungeon boss be the dragon i mentioned and an S rank dungeon final boss would be the demon king.