Looks like it'll be less tedious than SM. There's an encounter table for Poni Plains (probably a bush that Pokémon jump out at you from) that lists Petilil at a 50% rate, Scyther at 30%, and most notably Chansey at 20%, all of whom appear at levels 54-57. These Chansey will endlessly call more Chansey as well as the occasional Blissey, who has a 5% chance of holding a Lucky Egg. That ought to make levelling stuff up a bit faster.
Of course, you could just inject, but it's nice to know anyway.