Instructions
Head to the Guild headquarters the day after you defeat Primal Dialga for the morning roll call. Speak to all of the Pokemon in the town square and, per Wigglytuff's instructions during roll call, go to the Mystifying Forest.
Navigate the 13 floors of the Mystifying Forest dungeon, recruiting Pokemon along the way. Recruit a variety of different Pokemon to better prepare for battle. After Floor 13, move into the clearing and follow Ursaring and Teddiursa toward the Luminous Spring. Along the way, you will fall into a pit.
Select a fighting-type Pokemon to quickly knock out Loudred in the pit. Keep using Sleep Seeds to keep Wigglytuff out of the battle. "Earth Power" and "Vacuum Cut" are attacks that players can use against multiple opponents at once. An ice-type Pokemon's attacks can knock out Diglett, Dugtrio and Chatot. Use a ground-type Pokemon against Croagunk, a fire-type against Sunflora and a grass-type against Corphish.
Defeat Wigglytuff last, cycling between using attacks and items that have status effects that will keep Wigglytuff from fighting back. You proceed automatically to Luminous Spring, where you can open a chest and get the item "Perfect Apple." You can now access formerly locked dungeons, carry more items in your Treasure Bag and evolve Pokemon at Luminous Spring.