Update #3: The Potions
So. I don't know if I'm deserving of the title of Challenge
Mod after what I did in order to power through with this challenge, but technically... I did get into the hall of fame, right?
Let's just start with the Veilstone Galactic HQ, where I learned just how powerful Honchkrow can be. I was able to take care of Sneasel and Crobat with Lickilicky's Rollout (so fun to see that work) but Honchkrow needed some more work from Rita and Estelle, and there was really no Pokémon that could really take its hits that well. I got through Mars and Jupiter at Spear Pillar well enough but of course, you have to battle Cyrus again in the Distortion World! And I was starting to get underleveled. That's when I resorted to using lots of Hyper Potions in battle to give Judith and Estelle enough endurance to deal with his Pokémon, before Rita defeated Giratina and sort of saved the world.
That opens up one of my typical grinding spots for Sinnoh late game... Sunyshore and nearby routes. Route 223 has level 50 wild Pokémon (!!) and there's various trainers near there with Gyarados or other powerful Water types. The problem? Usually the best way to deal with them is Electric-types, and that's why I think they're so important. That's also why they're kinda rare in the list I submitted (although I probably should've been kind to myself and added Electrode or something). With a slow team and
Shock Wave Lickilicky of all things, grinding there was complicated, and even giving Duke Thunder Punch via Heart Scale could only get me so far. Luckily... I beat Volkner easily enough with Yuri's Dig (with help from Duke and Judith). Surely easier than anticipated.
The Victory Road trainers were also a bit easier than I expected, and that made me eager to just go all in with the league challenge. I probably shouldn't have done that so soon but... I just got some TMs, gave them to my Pokémon, and headed in. I'll already say, I didn't get to use those moves much... oops!
- Aaron was kind of complicated because of some of his switching and evasion strategies, but Judith got Rock Slide for this fight and put it to... relatively good use. Well, it still didn't do that much damage even to things like Vespiquen, but it got us through. Fire Blast from Rita destroyed Scizor and Duke dealt with Drapion with his new move, Earthquake, which replaced Thunder Punch.
- Bertha was probably the easiest, thanks to Flynn and Rita. Gliscor sure gave me some trouble because of Thunder Fang, but healing off the damage with potions was enough. Phew!
- Flint was a disaster to be fair. As it turns out, Yuri is somehow faster than his Houndoom and can defeat it with one Brick Break, but the rest of his team is way faster and more dangerous than my low level team can really handle. Throughout this whole battle, my hopes were on Duke, who eventually powered through with Earthquake damage and potions, whereas the rest of my team (including Rita) were mostly there to give me more turns to heal. I thought Rita could do better but the coverage moves that Flint has on his Pokémon ruined that plan, and... yes, this doesn't fit with my typical "fast Pokémon" playstyle.
- Lucian was finally Estelle's time to shine. Sorta. It was another sloppy battle, but luckily, I still had Sucker Punch on Yuri to deal with Alakazam, Rita with Fire Blast for Bronzong, and... lots of potions to sponge hits with Estelle as she politely bit into her opponents with Bug Bite.
- Cynthia... took me a lot of tries and money. There's no real reason for such a big level jump between Aaron and Cynthia's teams, but oh well... let's go. Spiritomb went down thanks to a Curse from Duke and Estelle just taking hits, but then Garchomp came in and my Wormadam succumbed. Rita could defeat Garchomp in two hits, but was always left with about 10 HP or so from her opponent's attacks, so I had to use Flynn there as punching bad to heal Rita back up. That's what you get for being weak to way too many coverage moves I guess. This is the point where I also resorted to use Revives and not just potions in battle, which allowed me to sponge all of Roserade's moves with Estelle and slowly whittle her down with Psychic. Milotic was the one Pokémon against which I had some sort of answer, Judith's Power Whip, so that was a change of pace. Lucario was a bit more threatening, but Yuri proved to be useful there and got his moment of spotlight, leaving just Togekiss still to go. Being Cynthia's last Pokémon and not knowing what to do (Lickilicky technically had Thunderbolt but, never was a TM more useless, courtesy of Aura Sphere on her opponent's moveset), I just decided to revive Duke one last time and Curse. Rita sponged the remaining hits until Togekiss went down, and... yeah, I guess I made it.
So... ok, I kept the Set battle style, and I was able to make it through underleveled, but... I used way too many items in battle, including revives. Feel free to roast me or fire me for that, idk :)
but pls have some mercy
Current team:

Estelle (Wormadam-Steel), ♀, lv. 51 @ Metal Coat
Anticipation - Psychic, Rain Dance, Bug Bite, Iron Head
My starter! She was actually not bad at all for the first part of the game, once I could get her evolved that is, and even later on she was surprisingly resistant. Her main advantage is obviously the typing (I'm glad I chose this form, for sure) but it has to be said... her movepool is quite limited and not learning anything better than Bug Bite doesn't help. I can definitely see why I skipped this Pokémon but after the reputation she's made for herself I'm actually very happy that I could take her to the league and have her hold her weight. It's a lovely redemption arc.

Yuri (Toxicroak), ♂, lv.52 @ Black Sludge
Dry Skin - Brick Break, Poison Jab, Dig, Sucker Punch
My impression of Toxicroak is that despite his power and versatility, it remains a difficult Pokémon to use well. As I already said, learning
Revenge as his main Fighting stab is the most glaring problem I could find him, but having Dry Skin and 85 Base Speed also should've made me act more considerate and careful when using this Pokémon. That said, that 150 Attack stat at level 50 was a great asset to the team.

Judith (Lickilicky), ♀, lv. 51 @ Rock Incense
Oblivious - Return, Rock Slide, Power Whip, Thunderbolt
I've actually been meaning to use more of these Normal types with large movepools but I admit, Lickilicky wasn't my first choice (that would've been Clefable most likely). In this case, Judith's Impish nature didn't really help her much with versatility either, and I feel like her level-up moveset didn't do her justice, save for Power Whip perhaps. That big HP stat and those nice defensive stats, though, really helped in many situations, and I'm not sure I would've made it all the way to the league this fast without her.

Duke (Dusknoir), ♂, lv. 53 @ Earth Plate
Pressure - Shadow Punch, Earthquake, Will-o-wisp, Curse
This is probably the last Pokémon that started becoming useful in my team. Up until level 40 or so, when I made him evolve into Dusknoir, he barely took part in battles and mostly leveled up with the Exp Share. But after that, he really turned into an important team member! Mostly because of Curse, Earthquake and that nice Attack stat - so, still kind of situational - but you know. That was essential, especially against Cynthia. Again, kind of a hard Pokémon to use in a general run, but not bad in itself.

Rita (Octillery), ♀, lv. 52 @ Mystic Water
Suction Cups - Surf, Ice Beam, Fire Blast, Psybeam
I feel like Octillery falls into the "good attacking stats but not much else" category with the likes of Exploud, Camerupt, Cacturne, Absol and more. Which makes it kind of a hard Pokémon to use once again, one that usually takes one or two hits before going down but can at least deal damage back before that happens. That has been the case actually, with Rita surviving many hits at low health, just enough to retaliate, and at least when it comes to deal damage... crazy coverage even by level-up moves (Ice Beam by level up? Amazing!) at least helped her. That's definitely the one thing Octillery had going for her. She may not be the best Pokémon but I still have to thank her for dealing with Cynthia's Garchomp, as well as Byron and a bunch of other Rock and Ground types~

Flynn (Tropius), ♂, lv. 50 @ Sky Plate
Solar Power - Grass Knot, Air Slash, Toxic, Body Slam
Admittedly, Tropius was a bit of a disappointment. A Grass-type without many of the moves that make Grass-types... Grass-types (Leech Seed etc.) and whose effectiveness as a Flying-type was often undermined by all the coverage moves that Pokémon usually had, and that happened to be effective against his Grass-type. I have to wonder if Jumpluff, despite its non-existant attacking stats, could've helped just as much here because of things like Sleep Powder. It did provide coverage in some battles though, and definitely helped against Bertha - plus, it's a single stage Pokémon, so back when I had recently caught him, he was a force to be reckoned with. So, once again, not so bad.
And there we go! I seriously need to take a break here, completing a challenge in 2 days or so was not properly a healthy thing to do ^^;