you must absolve the minigame cactus-game. You must go to the land of the cactus( sanubia- W-Part). Go into the sandstorm and activate the minigame.
Now you must collect all cactus in spira. Here are the places, where they are(they all in sanubia):

1) Oasis- besides the save-sphere
2) east-sanubia- in the NW, where is the treause "4x hi-potion"
3) West-sanubia- in the near of the plate, where something stand with 20%
4) middle- in the near of the trausre "10.000 gil"
5) east- in the near of the save-sphere( in the tent)
6) middle- SW in the treausre "2 mega-potion"
7) east- in the treasure with "3-final-elixiers". Before you go to this, you must have been in the middle-sanubia.
8) OAse- airship- go to the sphere besides the oasis. then he go away and you must go to your airship. here you go to the deck of the airship, where you fought vs. Efyre
9) West- you must go to the cactus-stone. The cactus is behind this stone

You must collect all cactus in this order. I hope i could you help.
MFG Clodan