the best place to train at is in enterna forest because when you with Cheryl run in the wild and battle Pokemon i got my ponyta up 6 levels in half an hour from lvl 20 to lvl 26 and you don't have to heal. Other places are route 202 route 217 and between velistone and the resort place just use the vs seeker and keep battling them like the collectors and breeders and others Hoped i helped :)
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Personally, I would train a Bird-Type Pokemon at a place where those types of Pokemon is very effective to it. Like Fighting-Types. They are weak to Bird-Types. So, I would train it there!
Let your Pokémon hold Soothe Bell and train OR feed it Berries that lower its stats but makes it friendly- DON'T overdo it.
Milotic
use wing attack [get starly and train to level 9 and it gets wing attack]
That all comes down to opinion. Everyone has different opinions about which is the strongest and which is the best.