I seem to remember the challenger page used to sort them by difficulty but now I can't tell the difference between a beefy medium and an easy hard. It would be helpful to know for tokens and stuff.

If you haven't gotten an answer yet, difficulty are listed in alphabet order. So, if a challenger is earlier in the alphabet then the last challenger, that's the first challenger in a new tier.

oh huh. that makes sense I guess. someone else linked to a spreadsheet, but you would think they would just like... label them on the official site. thank you!

Here's a good battle challenger resource! Comprehensive Guide to Battle Opponents (v2) It lists all the battle challengers by difficulty and also has tier recommendations.