You can go to Veilstone City and take your Chansey for a massage there. When its happiness is full, it will evolve by its ownself.
Hope I helped.
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Chansey > level up with max happiness > Blissey
Have the chansey's happiness be at the max to do that use protein, iron, carbos, hp up, calcium and zinc then level it up while holding a soothe bell it will become a blissey just like that.
One needs to increase Chansey's happiness to its maximum.
Chansey evolves when you increase its happiness. So beat loads of Pokemon with it without fainting, and give it loads of berries that increase its happiness. Good Luck.
chansey does not evolve by level. it evolves by happiness. i hope this helped!