>>36513473The inverse battle as soon as you reach Mauville City is extremely tough. He has 3 randomized level 20 Pokemon, around the same level your mons should be, and they all possess a ton of coverage moves and a held item. It's optional to do though and you don't white out for losing.
bulbapedia.bulbagarden.net/wiki/List_of_Inver's_Pokémon_in_Pokémon_Omega_Ruby_and_Alpha_Sapphire