On Cactuar Island you can find another secret GF called Jumbo Cactuar. The island can be found to the east of the Centra Ruins. You can only get there by Ragnarok. Just land on the island and you'll see a cactus popping out of the ground at a random location. When you catch it you'll have to fight Jumbo Cactuar.

Jumbo Cactuar
This boss is not really strong but it's 10,000 Needles attack wipes out a team member in one hit! He's weak against Water, so casting Leviathan and water magic is the best thing to do. When Jumbo Cactuar is on it's last legs a message will say that he's Hesitating. Stop with attacking and using magic, but keep using Leviathan. That will kill him!