World Champion is a Celestial Weapon and it is the most powerful weapon for Wakka in the game.
You must obtain the Celestial Mirror before you can obtain or create any of the Celestial Weapons. Click on the link below for more information on how to obtain the Celestial Mirror:
In order to create the fully powered-up World Champion you will need to obtain the World Champion weapon itself first, then use the 
Jupiter Crest and the Jupiter Sigil to power it up.
In order to obtain the actual weapon itself you must place at least third in a Blitzball tournament which you can plan at any time after your first trip through Luca. You must then speak to the Cafe owner in Luca with the Celestial Mirror in your possession and she will give you the weapon. The Cafe is located in Luca Square.
You will also need to play Blitzball to obtain the Jupiter Sigil and obtain Wakka’s Overdrives. Check out the Blitzball section for more information about developing a strong Blitzball team and strategies for winning individual matches:
The Jupiter Crest is found in a treasure chest in Luca. It can be obtained after the first Blitzball match involving Tidus, Wakka and the Besaid Aurochs. Enter the Besaid Aurochs locker room and find the treasure chest towards the back.
The Jupiter Sigil is a reward that you can earn for winning matches in Blitzball tournaments. Check out the Blitzball section for more information about the prizes available and the prize structure (for league and tournament games).
Return to Macalania Woods to the area where you received the Celestial Mirror once you have obtained both the Jupiter Crest and the Jupiter Sigil. Offer World Champion up twice to fully power it up using both the Crest and Sigil.
World Champion has the following abilities when fully powered up:
Break Damage Limit - Allows Wakka to do more than 9,999 damage in a single attack
Triple Overdrive - Charges Wakka’s Overdrive at triple the speed
Double AP - Allows Wakka to earn double the AP for each battle
Evade & Counter - Allows Wakka to counterattack physical attacks and gives the possibility of evading the damage as well. The counter will work regardless of whether the evade is successful.
