>>23325125Nothing special compared to Red or Barry
>>23325134Stephen is barely stronger than his Emerald team
Diantha is a joke compared to Cynthia, both as a Champion and bonus boss in Gen V.
No trainer in Gen VI can reach the same level of challenge as trainers in earlier games just because of how much easier it is for the player to get higher leveled better trained Pokémon faster than ever.
Diantha is strong in terms of levels but it's very easy for the player's team to be level 80 across the board by that point just by playing through the main game with the EXP Share on. Her team also has several crippling common weaknesses. Contrast with Cynthia whose levels the player will likely have to grind to match, has a solidly balanced team composition, and has perfectly bred and trained Pokémon that give the maximum possible challenge to the player.
Wally is good, mainly because he actually uses held items, but he is very much an exception to Gen VI's trends.