The best method is SOS chaining Chansey
First, have a Pokémon with false swipe and an adrenaline orb. This will make Chansey call more often.
Second, you’ll need a Pokémon that can OHKO chanseys. A high leveled Pokémon with physical attacks (especially if these attacks are fighting moves) should do the trick.
Having the Pokémon you want to train hold a Lucky Egg is very helpful, and using a Roto EXP points boost is also helpful. PP restoring items like Roto PP restore and Leppa Berries are also needed to keep the aforementioned Pokémon that can OHKO Chansey’s PP preserved.
Now, find a chansey. You can find them in the bushes at the Poni Plains.
Once you find it, false swipe it to 1HP and use an adrenaline orb. This Chansey will be used to call other chanseys. Bring out your Pokémon that can OHKO a Chansey and kill all the ones it’ll keep calling. In the turns it doesn’t call, use another adrenaline orb-it won’t consume the adrenaline orb, it will just fail and take up the turn so you don’t have to attack the caller Chansey.
After a little while, you may want to switch callers. This is because the current one might run out of PP and struggle to death. False swipe a called Chansey and kill the current caller Chansey.
Defeating Chanseys (and the Blisseys they can call) provides TONS of xp and it is the most efficient way to grind to level 100.
Hope this helps!