Roserade (Lv. 79)
Bright Powder (Ability: Poison Point) (Moves: Shadow Ball/Leech Seed/Petal Blizzard/Poison Jab)
Talonflame (Lv. 79)
Life Orb (Ability: Gale Wings) (Moves: Brave Bird/Flare Blitz/Steel Wing/Tailwind)
Azumarill (Lv. 79) @Assault Vest (Ability: Huge Power) (Moves: Aqua Jet/Play Rough/Aqua Tail/Iron Tail)
Magnezone (Lv. 79)
Air Balloon (Ability: Sturdy) (Moves: Thunder Wave/Discharge/Reflect/Flash Cannon)
Garchomp (Lv. 79)
Rocky Helmet (Ability: Rough Skin) (Moves: Dragon Claw/Crunch/Earthquake/Stone Edge)
Gallade (Lv. 81) @[~ 756] (Ability: Steadfast) (Moves: Psycho Cut/Close Combat/Leaf Blade/Swords Dance)
Changes:
Once again, the changes will be from his previous team above rather then something which was in Gen III.
1. Uses a full party of 6 Pokemon.
2. Swapped out Altaria, Delcatty for a Talonflame, Azumarill, and Garchomp.
3. All his Pokemon are now holding items (Gallade still has its Mega Stone).
4. Roserade is now holding a Bright Powder; lost Synthesis and Toxic; & gained Shadow Ball and Poison Jab.
5. Magnezone is now holding a Air Balloon; lost Screech and Tri Attack; & gained Thunder Wave and Reflect.
6. Gallade lost Slash and gained Leaf Blade.
Opinion:
Oh look, Wally is the first trainer I covered that remembered that a Pokemon can hold an item other then a Mega Stone!
Boy, what team. First out will be Roserade who's holding a Bright Powder so you may be missing it sometimes, probably giving it enough time to set up a Leech Seed. Talonflame is holding a Life Orb, has its Hidden Ability Gale Wings, and Brave Bird (though it may first try setting up a Tailwind); good luck with that. Azumarill is an offensive powerhouse who has high Special Defense, so use Physical attacks. Magnezone temporarily loses its quadruple Ground-type weakness thanks to Air Balloon and will probably try setting up a Reflect or paralyze our Pokemon with a Thunder Wave. Garhomp is another offensive powerhouse with a Rocky Helmet for your Physical attackers. Finally Gallade had took my advice and switched out Slash for Leaf Blade for more coverage.
So yeah, you'll be in for quite a battle here. But I'm curious now about these rematches. I'm beginning to wonder whether "Rematch 1" may actually be an initial post game battle with him and then later in the game, probably somewhere in the Battle Resort, you'll find him there and once every day he'll battle you; sort of like Barry in Gen DPP or Serena/Calem in XY.