Lucky Punch increases the critical-hit ratio of Chansey.
How to Obtain[]
Lucky Punch you can get in FireRed or LeafGreen on one of the Sevii Islands bring it to Diamond, Pearl, or Platinum by migrating them.
To Obain it in HeartGold and SoulSliver there is a girl on Route 15 and when you talk to her she asks you "Who is the cutest pokémon in the world... It's Chansey! I say Chansey and that's final! If you can bring me a Chansey I will be very happy!" If you put Chansey as your first pokémon and talk to the girl this time.."Who is the cutest pokémon in the world... It's Chansey! I say Chansey and that's final! Oh my... you have a Chansey! I'm so pleased! I never seen a chansey this close before! Here take this!" and you will obain the Lucky Punch.
You can get one in Black or White, there is a man at Route 13, after you talk to him, he can give you a rare item everyday, and it is possible to get a Lucky Punch.