If you are in post-game, I recommend training just outside the Battle Tree. You'll encounter Pokémon at around level 55 and you should be able to power level your Diglett. Make sure it's holding a Lucky Egg and has at least two hearts of affection (feed it a single rainbow bean). Lead with it and switch into a stronger Pokémon.
Alternatively you could battle the Elite Four once again to also make money and you have strong enough Pokémon to get you through.