For me, Shelly was the first Gym Leader I had trouble with- the illumise was a pain in the butt to get rid of before it used Rain Dance (note: I didn't stop it the time I won), yanmega is pretty strong that early in the game, and geez if the anorith's Telluric Seed wasn't annoying! Wound up having to overlevel+ Common Candy my starter so that my quilladin (now a mighty chesnaught) could take a few hits and give the other poke time to do the actual damage and hunt down a Telluric Seed of my own- and even then, it still took me a few more tries to beat her!
I actually don't remember how much trouble Shade's gengar did or did not give me, but I do remember having a fair bit of trouble with the rotom (mostly because I really needed the field to be in Factory mode when the true devil came out, and Shade isn't always compliant with his Discharges). That mimikyu, though... That was quite the roadblock on it's own accord!
Adrienn has the dubious honor of being the only Gym Leader I trained a pokemon specifically for and had it actually work. Xe refused to blow away the Misty Terrain, so I decided to just do it myself. Bonus Points: a Max Revive was absolutely essential to my strategy for actually getting past xem. That says quite a bit about the battle, no?
Titania gave me just as many problems as she seems to have given everyone else. It probably didn't help me any that I was kinda underleveled going up against her and trying to grind in Never After seemed to be quite the hassle. I'm not gonna lie, though, my victory actually caught me completely off-guard; my flygon (my last pokemon) had terrible track record in terms of surviving enemy attacks (low IVs will do that to ya), and Titania's significantly-higher-levelled-than-your-current-cap aegislash had +3 Atk. under its buckler- and yet the flygon managed to hold on and deliver the killing blow anyway!
And lastly... Amaria. There are... levels to this madness. To begin with, I'm running the Reshiram route, so I had all the emotional shenaniganry associated with that to put me a little off-balance on my first try. This also means that she used Mega Evolution against me rather than the Z-Moves she uses in the Zekrom route- that's significantly harder to prevent and potentially far more devastating to go up against, no? This also means that she uses the team opposite of what you tell her you prefer- a detail I did not pick up on at all until after I had quit the first time as it led me to believe that she forced you into playing Doubles regardless. The fact that her Doubles team is ridiculously brutal didn't help; leading with two twice-Field-boosted Surf spammers with Water Absorb is an absolutely ridiculous start, especially since one's packing a Life Orb to capitalize on all the free healing, and the other has friggin' Rain Dance on its moveset! Out of nearly 20 attempts, I had only managed to take down a single pokemon one time. ...Two months of tarrying later, I finally built up the nerve to try again, this time using what I learned about manipulating which battle style she uses against her. On one hand, she's much easier to deal with in singles (I took down half her team on the first try). On the other hand... Dual Screens+Vollt Switch right out the gate, backed by perfect-accuracy Hydro Pumps? A significant number of Swift Swimmers on a Field that lowers non-Water pokemon's Speed already? The potential to give already bulky pokemon a free (boosted) Aqua Ring and a Sp. Def. if you time things poorly? Three friggin' Ultra potions?! Goodness, she has a lot going for her this time, and none of her pokemon even have Surf! It took me a whole day's worth of poking and prodding, as well as spitting on color theory enough to make poor Indra cry, to finally pull out ahead and claim my goddang Badge, and even than Iwould've lost yet again if she hadn't opted to waste her last turn trying to heal up Deliverance.
So, uh, yeah, those are all the Reborn Gym Leaders that gave me a significant amount of trouble that wasn't simply because of a cheap level spike (glares at Samson) so far. Sorry about the text wall...
