First, you must have the Ragnarok, five Elixirs, and Enc-None (Diablos). Now, go to these followiung places with Enc-None equipped, and run around. You’ll see a UFO.
Next, land the Ragnarok near a ledge overlooking the Chocobo Forest in Grandidi Forest. Run around, and fight the UFO. After killing it, equip the item command and return to Balamb. Run around where the garden was and run into PuPu. Either kill it (Accelator) or use five Elixirs on it for the rarest card: The PuPu card.