For making tiers of difficulty, I would suggest something along the lines of what Final Fantasy XIV does for their boss difficulty modes. Typically there are 3 difficulties (a Story/Normal "You should be able to clear this blind", a Hard "Probably won't clear first try, but should be good to clear today"and an Extreme "You need a dedicated group, don't expect to clear it until you've got it down to a science") Each level of difficulty adds more mechanics or phases or special attacks that make each fight very similar, but each layer does add a lot to remember.Best part, regardless of level, most of these fights are in-and-out in maybe 5-10 minutes. (there are exceptions) *Edit for FFXIV players, yes I am aware "Extreme" fights made at launch are now laughable. My description is referring to how difficult the fights are at their introduction.