Ok, so here's what I think! When you go to the arena, there will be another guy who lets you fight in a match: only with normal enemies, for a little quick challenge! So there will be a screen that says which enemy you want. Every single one (that you have already fought once at least) will be listed. You will need to defeat a certain amount of bosses to fight to complete the challenge.
good idea, like if i wanted to fight gurtok demon for practice, i could fight him alone, but you should still get drops. I also think you need to rank up in order to fight harder ones, even though you have fought and beaten them before. put into consideration KI
