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You can encounter Chansey 20% of the time in Poni Plains, as opposed to past games where Chansey was hidden behind a different Pokemon in SOS battles.
Chansey is able to SOS call both itself and Blissey in SOS battles in Poni Plains.
Exp. Share + Roto Loto for EXP + Refresh buffs = Faster level up system
Granted this is post-game but it's an excellent method to level up 6 Pokemon at once. It's not as fast as Blissey bases mind you but there is no daily limit either.
Keep on killing Blissey and using the aforementioned EXP buffs, and you will eventually get to that point.
Oh, and you can also farm Lucky Eggs easier now if you have a Compound Eyes Pokemon in the top of your party and use Thief.