Instructions
Defeat the Elite Four. Go to the S.S. Aqua Ship in Olivine City to receive the National Pokedex. Travel to the Safari Zone. Once you gain the National Pokedex, more areas of the Safari Zone open up to the player.
Speak to the Safari Zone Warden, Baoba, who asks you to bring him a Geodude. Capture Geodude and show it to Baoba. This allows you to customize the Safari Zone. You will be able to move the different areas of the Safari Zone around. Place the Marshland region closer, so you do not have to waste steps.
Capture a Sandshrew for Baoba when requested and show it to him. He allows you to bring objects into the Safari Zone. This allows you to place block items in the different regions of the Safari Zone. Baoba calls you regarding block items that can be placed throughout the different Safari Zone regions.
Take block items that have an effect on Forest areas. These block items include trees, stumps and branches. Travel to the Safari Zone's Marshland Area. Press "A" to drop your block items. You will need to drop a total of 21 block items. Wait more than 20 days, allowing all of your block items to level up.
Roam the Marshland region until you face Croagunk in a random encounter. Use a Safari Ball to capture Croagunk.