Hmm.. I don't think this should be overthinked too much.
First as I see it, we want to see, fun, funny and interesting (some of them) fights.
About planning oponnets, I just mentioned ideas for interesting duals- parody Luffy vs Masala- because of their stretching powers.

Lucifer, just like Satan has an epic plot armor, and every battle against him would be crazy and fun.
But I don't want him to win the tournament I want him to win one or twice, and the Frozen's character go Chi Chi on him and kidnap him to a church and marry him.

Fenrir- simply want to train to be a good leader. (And eventually king of earth?)

Black Sultan should be an interesting swordman and sand bender that is cool to beat.