Instructions
Play the game until you have encountered and caught a Chansey. They sometimes swarm on Route 13 which gives you better odds of catching one or you can search for one in the Safari Zone. There is also a small chance of catching a Chansey along Route 15.
Put the Chansey in your active party then travel to Route 14. Go up to the area where the grass is and use the "Cut" move to get past the tree.
Talk to the person found here who asks to see Chansey. Show him your Pokemon and you receive the "Lucky Punch" as a reward.