MeonOne's guide is somewhat correct.
You have to complete all 5 objectives on the level and get hit with every pie thrown at you.
I used the level "Rootin' Tootin' Shootin'"
1. Earn 111,000 Points (self explanatory)
2. Knock down all cacti (there are 4 cacti as soon as the level starts on the bottom of the screen, one in front of the train house, one to the right of the RR crossing sign, one to the left of the RR crossing sign, and one to the left of the mail house.
3. Shoot down 3 buzzards (almost impossible to miss, they fly across the top of the screen every few seconds)
4. Shoot 3 robbers ( About 15 seconds into the level your first robber will appear. He will be to the left of the bank/safe. Second one will appear on top of the bank/safe. The third will appear as a moving target above the bank/safe)
5. Fill the barrel with fish (shoot the 100 point target underneath the water tower. Next, shoot the three 500 point targets)
Hope this helps :)