Here are a few things you can do
-Change Chansey's type so it loses STAB.
-Use reflect with Shuckle.
-Make sure Chansey is burned to lower its attack.
-Make a high level difference. A level 1 Chansey against a level 100 Shuckle.
Remember that the game doesn't register decimals for damage calculations. It will always round up if the value is less than one, making that the least amount of damage you can do to a pokemon.