Roxanne - Rock
Team - Anorith, Lileep, Nosepass.
A fairly balanced team which will have super effective attacks against all three starters.
Roxie - Poison
Team - Koffing, Arbok, Scolipede
Note the Scolipede has Speed Boost.
Faulkner - Flying
Team - Gyardos, Fearow, Aerodactyl, Pidgeot
An Electric type will do well here hence why Faulkner carries an Aerodactyl.
Volkner - Electric
Team - Jolteon, Alolan Raichu, Emolga, Electivire
Some may think this is too low for what was an 8th Gym Leader but I think this is about right. Note things get harder after this.
Valerie - Fairy
Team - Sylveon, Clefable, Klefki, Aromatisse, Mega Gardevoir
Your first battle with a Mega Pokemon. Note Sylveon has Pixillate.
Candice - Ice
Team - Glaceon, Lapras, Alolan Ninetales, Weavile, Mega Abomasnow.
Opens with her Mega and Alolan Ninetales has Aurora Veil.
Clay - Ground
Team - Hippowdon, Krookodile, Swampert, Excadrill, Mega Steelix
Note Hippowdon has Sand Stream and Excadrill has Sand Rush.
Clair - Dragon
Team - Dragonair, Salamence, Haxorus, Flygon, Kingdra, Mega Charizard X.
A very strong team to make sure you are ready for the Pokemon League.
Elite Four
Flannery - Fire
Team - Ninetales, Torkoal, Charizard, Magcargo, Delphox, Mega Blaziken
Both her Ninetales and Torkoal have Drought and Charizard has Solar Power.
Erika - Grass
Team - Whimsicott, Lilligant, Venusaur, Vileplume, Tangrowth, Mega Sceptile.
Whimsicott will cast Sunny Day first turn and has a Focus Sash to make sure. Lilligant, Venusaur and Vileplume all have Chlorophyll. Lilligant also carries Sunny Day. Mega Sceptile rounds out the team.
Misty - Water
Team - Gyardos, Blastoise, Tentacruel, Vaporeon, Starmie, Mega Swampert
All Pokemon except Mega Swampert carry Rain Dance.
Marshal - Fighting
Team - Conkeldurr, Machamp, Mienshao, Scrafty, Terrakion, Mega Lucario.
The Fighting Master gives the player a straight up challenge with his main Pokemon being Mega Lucario. He also appears to have befriended a Terrakion on his travels.
After you have defeated the Elite Four Marshal will tell you the Champion is a person he knows well and has the utmost respect for. You head through the door and see
Champion Alder.
Team - Reuniclus, Bisharp, Chandelure, Volcarona, Mega Heracross, Reshiram/Zekrom (Version dependant)
His Pokemon represent each member of the original Elite Four:
Reuniclus - Represents Caitlin. Has a Mental Herb to prevent being taunted and Trick Room to turn his largely slow team to his advantage.
Bisharp - Represents Grimsley
Chandelure - Represents Shauntel
Mega Heracross - Representing both his pupil Marshal and his own love of bug types
Volcarona - His signature Pokemon
Unova Dragon - Representing he was originally Champion of Unova.
If you defeat him he will induct you into the Hall Of Fame, give a congratulatory speech and give you a key to catch the other Unova Dragon.