Details:
Before you can challenge Wattson, you will have to face Wally who believes he has gotten well enough to begin a quest similar to your own. His Ralts isn't very powerful so this shouldn't be a hard battle.

Pokemon Trainer Wally	Ralts
Battle Type Items	Level 16
	Attacks: Growl Confusion Double Team Teleport
	Hold Item: No Item

Details:
You don't see Wally again until the end of Victory Road. He challenges you with a well rounded team this time. Be careful as this can be a very taxing fight before you can get to a Pokemon Center.

Pokemon Trainer Wally	Altaria	Delcatty	Roselia	Magneton	Gardevoir
Battle Type Items	Level 44	Level 43	Level 44	Level 41	Level 45
	Attacks: Aerial Ace Safeguard Dragon Breath Dragon Dance	Attacks: Sing Assist Charm Feint Attack	Attacks: Magical Leaf Leech Seed Giga Drain Toxic	Attacks: Supersonic Thunderbolt Tri Attack Screech	Attacks: Double Team Calm Mind Psychic Future Sight
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

After beating the Pokemon League Wally can be refought once per day. On each new challenge his team's levels grow by 3 until maxing out at 12 levels above what they were the first time you fought him on Victory Road.

Pokemon Trainer Wally	Altaria	Delcatty	Roselia	Magneton	Gardevoir
Battle Type Items	Level 56	Level 55	Level 56	Level 53	Level 57
	Attacks: Aerial Ace Safeguard Dragon Breath Dragon Dance	Attacks: Sing Assist Charm Feint Attack	Attacks: Magical Leaf Leech Seed Giga Drain Toxic	Attacks: Supersonic Thunderbolt Tri Attack Screech	Attacks: Double Team Calm Mind Psychic Future Sight
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item