First off, I'm surprised Charlotte is such an issue for so many. I would have guessed she would count as one of the weakest.
Speaking of nerfs, I can tell a few leaders are weaker than before. Noticeably Florinia, but funnily enough I don't remember her e16 team, assuming that was pre-nerf.
I've done 3 playthroughs so far. My first playthrough was blessed with lots of shinys, but a Blaziken cursed with a Serious Nature and the weather bug. Rain. Rain everywhere. Pain, sorrow. Frustration. Hatred. I cheered when Amethyst got the Hydreigon treatment. A little. Just a little.
So, my virgin voyage into Reborn e16:
Julia: scary. It reminded me of my all-time most difficult leader to defeat in my first game, Sapphire, Wattson. Only instead of a magnemite spammer, it was a volterb Boom festival. But It did it. Somehow, I figured out how I could trick most of her Voltorbs into self-destructing on their own. Then it was just a matter of luck to whittle down Electrode. Espurr was the answer to my dutiful and desperate prayers. I couldn't have survived without that thing. 7-10 tries at least.
Florinia: I don't rightly recall her team, which leads me to suspect I beat her handily way back then. Seeds weren't a thing, and I know her Maractus was not impressing me. But before Reborn, I was a casual player. And when I mean casual, I mean CASUAL-i-don't-even-know-what-Ev-training-or-Natureswhatarethoselol-even kind of casual. Despite that, I did pretty good all things considered. So maybe she took three or four tries? It was about 10 or so tries, I think.
Taka: It was raining again. In a forest. Blaziken was not going to cut it that time. I got beaten rather handily by Taka a few times. Took at least 3-5 tries.
Corey: Someone posted that he was Reborn's Gatekeeper. Well, for me, he definitely was. That leader was tough. I had to train for some time outside the gym before I could best him, and that was in the regular field, due to me not finding one of the officers in a cave, I do believe. I prefer the storyline for this version of events, though, so I don't mind. His team seemed more than frightening enough. Blaziken was NOT going to beat Crowbat, and my Meowstick, nicknamed Esspuroyal, who I expected to do short work of him, didn't handle Skuntank so well. My poor, poor Granbull, too. XD At least 10+ tries. Or at least so many tries I stopped counting or even bothering to restart my game. Was a broke vagabond by the time he died. I guess Corey or Heather got all his prize money in the end! And when he offed himself... I was like "I am going to have to fight ANOTHER poison gym leader aren't I... in a HARDER field..." and then Fern showed up to make everything worse. Edit: forgot that I trained a Skorupi to a Drapion for that fight. I think that's what helped carry the win in the end.
Shelly: Reborn's MVP. I CANNOT. I repeat, CANNOT believe I failed to vote for her. I think the trauma just sapped the memory of my losses out the window when I looked at the polls. I ended up choosing two trainers who really don't deserve the vote. Oh well. Everyone who plays the game will learn to their own pain. Shelly is a monstrosity. And e17 has just made her better, in my opinion, or no worse at least.
Where do I begin?
Right, the part where I sent in Blaziken and got devastated instantly with no warning. I was expecting someone tough. Her gym puzzle scared me. Her team scared me more. That girl put the fear of Bugs into this trainer, and into my poor Blaziken, who will never feel like a real phoenix martial artist rooster. Vulbeat and Illumise comboed to take down my special defense. Masquerain's rain dance+bubblebeam+intimidate slaughtered Blaizken's physical presence. Anorith was either a complete miss, or The Wall. I don't think Yanmega gave me much trouble. But by then it was so close she could still end it, especially if she'd switched out Masquerain before I Ko'd the hateful thing. I know I lost at least 20 times with her.
My team at that time was Granbull, Esspuroyal, Blaziken, Pelipper and Drapion. I had something else, but I don't recall what it was.
Shade: First time around, he quashed me, like a bug, reminding me of the previous gym battle. Drapion's Night Slash proved invaluable here. I recall him being the easiest leader I'd ever encountered, much to my surprise. Easily did him in around the 5-6 try. Or less.
Pulse Muk: this thing... this thing counted as a wall. I almost quit the game. It almost too much. But I got lucky and somehow KO'd it with either Drapion or Pilipper. Somehow. Assuming Esspuroyal didn't just fix the problem as always.
Cal: It was a very hard fight for me in my first playthrough. Very hard, actually. It took me 5 tries at least to overwhelm him. That last try was raining. Rain+Pelipper=win.
Kiki: easy as could be. So I thought. Still caught me off guard because of the field. But Esspuroyal and Granbull dealt more than fatal blows. Really, everything on my team either neutrally resisted her or was effective against her, save my newcomer, a Shiny Klang.
Fern at the Wasteland Hideout: It never occurred to me that I might want to heal before seeing the rest of the "gym". Fern ambushed my weakened team and swept it. Then he swept it again. And again... and again. I am not totally sure what went wrong with this fight. My original playthrough saw all my rivals, Cain, Victoria and Fern, easily beaten in each battle... I had come to disregard them as cheep experience leveling. Partly because Granbull and Esspuroyal, and a lesser extent Pelipper, matched their teams perfectly. Not sure. But I got him on the sixth or so try.
Aya: Ouch. Hard. Definitely a wall. A terrible, painful wall. She was almost as bad as Shelly. Almost. I forget how I ended up pulling a win. It was a desperate struggle though, and that included plenty of restorative items and such. But wow, I was impressed by how much harder she was than Corey and Cain. It was likely the use of Doubles and the wasteland combined... took about 7 or so tries.
Serra: *Starts cackling like a madman. Laughter devolves into shrieks and curses, amidst shattering glass* This was the second time I really contemplated quitting. I was under a pathetic, and pleasantly misinformed, notion that Radomas was the furthest I could go. And I was pretty sure I was closing in on his gym. So I decided to keep going until the end. What do you know, but that was one heck of a gym battle. Took 20 times at least to finally break her illusions. Honorable mention to Bennett for trying to pass himself off as Shelly. Nope, sorry, kid.
Noel: easy. Took only 2-3 tries before I smashed him into the flowerbeds with High-jump Kick.
Radomas: surprisingly, was easy enough. I was disappointed with his team and it went down rather fast once I figured out what to do. And Klinklang was most helpful here, I think, but I really didn't get a lasting impression on Radomus' battle style. Future playthroughs would have differing results.
Luna: first time battling her was intense. I don't rightly recall why or how, but her Malamar was always somehow able to set up with Superpower. that made it much more frightening than I was expecting, and Blaziken seemed to be the weakest Blaziken I've ever trained. I legitimately think my defensive-ev trained (by total accident) Blaziken in my Sapphire playthroughs is better than an offensive EV specialized build. But of course, there IS the issue of power creep that Reborn experiences that static official games do not. But I digressed into a rabbit hole. How proud would the dreamer be? Beat her at last after rearranging stuff. I think this is where my Granbull played a prominent role.
Archeus: what Archeus? Blaziken won the day that time.
Samson: proved scary. I ended up switching up my team, taking on Gardevoir to replace Esspuroyal and Granbull, and Pelipper with Dusknoir. I also picked up Clefable. His Howlucha was an unstoppable juggernaut for me until I used that one incense that improves evasion+Minimize with Clefable. From there, it was an easy sweep. 2-4 tries.
Charlotte: Easy. I really don't get what the problem was for other players. But I am probably guilty of overdoses of revives. I also happened to go on a water type binge and ended up with a Kingdra and Clawitzer before fighting her. That probably helped... easy 2-3 tries until victory was achieved. It also helped to have an untouchable Clefable and a hard-to-hit Dusknoir. Curse and Double Team works nicely. And Dusknoir and Clefable are just plain scary when working in tandem.
Blake: I know, I was surprised too. He pwned me numerous times. I was totally stunned to get beaten on the first try. And then the second, and the third, and the fourth, right on down to the fifteenth or so try. So 10-15 tries with him. It was his Frolass that always did me in. I was pretty miffed that it got taken from his team in e17. Though my e17 playthrough showed how the new team can really cause trouble as well.
Terrawr: I really was expecting a challenge. Instead, I found her insanely easy. That fight went well, and so I lost fear of Garchomp from then on. Thank you, Gardevoir and Clefable. Fairies are beasts. All Hail the Pink Overlords of Typing. And some help from Kingdra and Clawitzer.
Fiore Mansion: People had trouble with this? Really? Not me. I lost the first try, but then made a simple transition to ensure Clefable or Dusknoir were out first. They acted to stall until a proper strategy could be implemented, and then my attackers proceeded to sweep. 1-2 tries. Then I went through my 3rd playthrough of Reborn. Ouch. Could not stop losing! Julia is a much harder team mate than Florinia.
Ciel: Oh my Archeus. She nigh broke me. And then I gave up and used the Minimize Tactic again with Clefable. Insta-win. 10-12 tries.
Adrienn: Sorry, Adrienn, you were just a joke. I'm sorry, Ame, but giving her a field that promotes Steal type usage is... not wise. My Klinklang and Clefable+Meteor Mash tore xier team to pieces in seconds. 1st try win.
Titania: Hmm, I actually ended up playing with my second playthrough team first, and then with my original team. The second game team was the superior one, and yet it fared badly against Titania. I lost several times before achieving victory. My original team did phenomenal, from what I recall, taking only a couple tries to acheive success. 6-7 tries / 2-3 tries until victory.
Amaria: *Shudders* Yes, Ameria hurt. My first playthrough team did well enough, but ONLY against her in singles. I was so sure my Magnezone would win the day. I was wrong. Dead, dead wrong. My second playthrough team did worse, as I was determined to actually win under water. That proved... unfortunate. Eventually I won with Rotom Wash in singles. Somehow. 15+ times to win.
My intention was to list my second playthrough battles separately, but that's too long and pointless. Suffice to say I actually had significant issue with gyms I hadn't previously, with the exception of Shelly, who proved almost invincible... and some special cases below:
Bennett: I don't know why, but in my second playthrough, Bennett was immortal. He simply could not go down to my team. I had used a mod to give me Larvesta as a starter. Yeah, that wasn't fun. Not at all... I even had a Metang, of all things. It still wasn't enough. He still crushed me every darn time. Easily went 10+ times getting stomped. No wonder Elias told him he had potential!
Serra: I caved and used my Donphan to use earthquake and break the field. It was simply not going to happen with my team, period. At all. Bennett had been insanely hard, Serra was mirror hell. It was break it or rage quit forever.
Noel: my second play-through turned this kid into a monster. I don't understand rightly why. 12+ times at least to win.
For the e17 play, I ended up bringing in a team of mons from the other games, all at appropriate levels, of course, because I hate starting out with basic teams. Training different Pokemon I love is one reason I like playing... and I wanted as many Gen 7 pokemon as possible. Florinia was definitely easier this time, as was Julia, though she still gave me some trouble at the start. Charmander was not treated kindly by her electric types. I recall Aya being likewise disposable. Her using Toxapex was too predictable. I'm not sure what it replaced, but it was a poor choice. Maybe if it made better use of spikes and toxic spikes. That's how I ended up winning, spamming spikes with my Skarmory and Toxapex until victory was mine.
Shelly: of course Shelly. Raindance+Araquinid is an overpowered terror. Combine that with her team members, and she's just plain scary.
Noel: again, not sure why, but he overwhelmed me here. He was an unexpected problem after having struck down the likes of Serra with ease. It was likely Skarmory's doing once again that I managed to pull a win.
Radomus: he proved a surprise challenge. Funny how his field renders Bug types the best type for him. I had an Araquinid and Vikavolt. Those chess pieces smooshed them, and Toxapex was as nothing before his psychic types. Skarmory eventually won out, I think, though it may have also been the assistance of my Charizard setting up Dragon Dance several times that did it in the end. 10+ until victory.
Adrienn: in the third playthrough, Adrienn actually dealt me some significant trouble. She actually shifted her field around, for starters. That made it harder to deal the Steal weakness, and she was apparently able to counter poison types rather well, something previous playthroughs didn't reveal to me. I was impressed. 5 losses until victory.
Ameria: she gets an honorable mention because I beat her underwater at last. By the power of Alolan Exeggutor! I swear, that thing is stronger than I ever dared give it credit for. I have no doubt I could have obliterated her singles team.
Speaking of which, my original two battles with Ameria were from the community, not public release. She did not have a Wishiwashi in those battles, and didn't in the Doubles. What made her Wishiwashi so tough? And what did it replace?
How do I get to the water in North Adventurine Woods? There doesn't appear to be a means of getting past those spider webs.