Chansey is beast wit evolite, to put it first. With a 105 base SDef stat and a 250 base HP stat it is a feared SAtk wall. Holding an evolite and with moves like Softboiled and refresh, it makes those SAtk users want to bang their heads on any hard object. Evolite will boost Chansey's measly 5 base Def, so Chansey won't heavily demolished by any physical attack, not to mention any physical fighting move.
I suggest this basic moveset:
Ice Beam/Egg Bomb
Suggested EVs: 252 SAtk / 252 Spd / 4 Def
Suggested Nature: Timid or Modest
Anyway, I hope I helped.