Easy Mode
Do you actually need help for easy mode?
Normal Mode
Orion - He is basically a sun leader. D. Charmeleon resists most of his attacks, but that isn’t enough to take him down. Onix with Sturdy can get rid of the Charmeleon he has and can set up Stealth Rocks that make the fight much easier, and Ponyta takes advantage of the sun to take out Deerling & Ivysaur. For Vulpix, use Onix, since Sturdy allows him to take a HP Grass.
Xavier - A good & fast Psychic type such as Kadabra or Gardevoir takes out 2/3 of his team. That beedrill is an issue. A strong psychic type could take it out, but a focus sash would be needed. For that shuckle, it isn’t hard, since it has NO HP bulk. Any good rock or steel type gets rid of this pokemon easily.
East - A good ice type takes out 1/2 of the team. A good rock type decimates Talonflame and Taunt & Thunder/Fire attack gets rid of Skarmory & Freeze-Dry users and Fairy-Types get rid of Kingdra easily.
Harmony- I recommend using Delta Vespiquen (with levitate) as it resists all of her Mons STAB Moves and can also do some serious damage to her furry (kek) type mons. For Devil Mega Milktank, A Strong Fighting Type can pretty much obliterate it, I recommend Hawlucha
-GiaPe
Anastasia - Delta Muk SHINES in this gym with Regurgitation, doing well against 4/6 of her mons, but gets taken down by Metagross EASILY. A good grass type takes care of Rotom Wash, and for Metagross use Talonflame. Set up with swords dance first turn it is out (holding a Focus Sash, of course) then use flare blitz next turn. (+2 0 Atk Talonflame Flare Blitz vs. 0 HP / 4 Def Metagross-Mega: 306-360 (101.6 - 119.6%) – guaranteed OHKO) The recoil from Flare Blitz won’t matter, because Talonflame won’t be used for the rest of the battle.
Diana - In my opinion, one of the easiest gym leaders. Gengar shreds through most of her team. A good fairy type gets rid of Sableye easily, and Mold Breaker mons with X-Scissor tear through Mega Gothitelle.
Calreath - Mega Delta Typhlosion MURDERS in this gym. I suggest you use it, because it has moves to take down EVERY. SINGLE. POKEMON. Thunder for Politoed and Manaphy, Energy Ball or Thunder for Kabutops, Dazzling Gleam for Dragonite and Goodra, and some strong coverage move for Heliolisk, since he can’t hit it with any super effective special moves
Adam - Fairy Types arent as good in this gym as you thought, thanks to Noctem lowering the power of their moves. A fighting type like Mega Lopunny shreds through most of the gym. A good ground type takes drapion, and Spiritomb? That could be a problem…but just use some strong STAB’s to take it out.
Hard Mode
Orion- Spamming Calm Mind on D. Ivysaur (Make sure to evolve it at level 25 so it learns Calm Mind before the level cap) can easily wipe him
Xavier- starting off with a pokemon with rapid spin will be useful starmie can easily beat his D Metang. but his team is incredibly fast so getting a fast or bulky pokemon WILL BE OP. Double Team + Nuzzle emolga(The great @PeterHolmes74 has been referenced) from the Friend Safari will be useful (Username: Akos)
East: You might feel bad for ending his winning streak/his life(spoiler) but you’ll need to win MD Venusaur’s Hubris is BrOkEn. For skarmory Rapid Spin Starmie is here to remove Spikes and Stealth Rocks and Kingdra wastes a turn to set up rain, so Mega D. Venu + Calm Mind can probably sweep the rest of his team except for Talonflame and Gliscor. A good rock type move takes out talonflame, andl D. Scyther can stop Gliscor with his strong Ice STAB.
Harmony: Steel types shine brighter than the Sun here using a noctem pokemon like Mega D. Zard is little helpful but wont do anything without a steel type on the team her only normal type is miltank, and DAMN i never thought curse and scrappy would be that useful…sigh. Don’t let anyone on her team use Belly Drum/Curse because Azumarril +6 Atk aqua jet could end anything in it’s way . SO anyway a good steel type like Delta Vespiqueen can sweep through her team but dont start with her start with a pokemon with priority move for me I used Aqua Jet D. haxorus with +2 Atk.