First off, I really need to say, I absolutely loved this hack. I had so much fun on multiple playthroughs, and decided to challenge myself. I was going to beat the Elite 4 with 18 different mono type teams, with no overlaps, and no legendaries. I also didn't use any mons that were considered Ubers, except for Darmanitan-G for the Ice challenge. It was pretty hard to make a team as it was, and I already used Ninetails-A for the Fairy run (in hindsight I should have just saved Ninetails for Ice and used Meganium instead). And today, I am proud to say that I finished it! I did most of them without saving inbetween members, but for some of them that was like literally impossible (looking at you Rock and Ice trying to beat Bruno...).
If anyone wants to know about how I did it, I might do a detailed run though of one of them. Rock was definitely the hardest, and Normal was by far the easiest, but Psychic and Water were pretty easy as well. My favorite team has to be the Grass one with Choice Band Rillaboom with Grassy Glide is an absolute house. Some unexpected powerhouses include Ditto from the Normal team (copying Ubers is awesome), Shedinja (several pokes literally can't touch it), and Choice Scarf Girafarig with Parental Bond and Boom Burst.
In general, you really want like 5 things. Priority, a big meat shield, a check for Zacian-C, Primal Groudon and Primal Dialga, and like loads and loads of speed. Timid>Modest, Jolly>Adamant basically always. Stall is really effective, especially later in the fights.
Don't bother with hazards, they really don't work. Lorelai is doubles, so they don't matter much, and her only focus sash user is the lead Ludicolo. For Bruno, both his leads will basically always taunt you, so you can use that to your advantage. Agatha will always taunt on the lead as well. For Lance, Aerodactyl will always taunt, and Garchomp is 50/50 Stealth Rock or attack. For the Rival, Krookodile will always taunt as well unless you have Magic Bounce or something like that.
Also don't bother with more than 1 setup mon. There are really only 2 reliable places to setup, and thats against Bruno's Bisharp trying to Sucker Punch, and against the Rival's Ditto. Occasionally Lance's Garchomp will just Swords Dance in your face but thats rare.
In general, Lorelai's Ice team was easier to beat than Rain, but it really depended on what team I was using. For Bruno, the Infernape lead team was generally easier. For Agatha, both teams are basically the same to me. For Lance, I preferred to face the Garchomp lead, but they were both relatively easy to play around. For the Rival, you can cheese the Krookodile quite easily. If your lead has a non-attack move, he will always taunt, so you can just attack. Then if you outspeed he will almost always switch to Groudon or occasionally Togekiss or Darkrai. You can cheese the Ditto quite easily as well, its basically setup fodder for a bunch of mons.
Weather and Terrain teams are very good here, especially against Lorelai, so use them if you can.
If at all possible, I would love to be able to rematch the Elite Four again at level 100, or at least not have the entrance blocked off. Having to reset my game everytime I completed the challenge was quite cumbersome. Some other things that would be absolutely lovely late game are more reliable ways to get competitive items, as the 5% chance drop of choice items from select Raid Battles is very low. Also, accidentally using a rare candy on the wrong pokemon and getting them above level 85 is very cumbersome, and should maybe not be allowed to happen if you're going to keep the cap at 85.