The secret fruit stamp.
Ingame stamp description:
A Secret to Everybody: There is a secret target in every stage of every difficulty level. Hit all the secret targets in every level to get this stamp.
Strategy:
It helps to use the environment as markers to help you adjust your shots until you finally get them right. Like, I would take note of how much higher my reticle is than a cliff or whatever, and adjust from there.
Difficulty: 3/5
Wii Sports Resort All Stamps Playlist:
• Wii Sports Resort (Wii) | All Stamps