Wakka Celestial Weapon - World Champion
If you didn't already know, in order to complete any of the Celestial Weapon quests in Final Fantasy X you'll have to first obtain the Celestial Mirror. Once you've obtained the Celestial Mirror you'll be able to open the chests that contain each character's Celestial Weapon as well as power them up in Macalania Woods.
Wakka's Celestial Weapon is called the World Champion and if you haven't already figured it out, it's going to take a lot of Blitzball to acquire this. First up, to acquire the World Champion you'll need to place at least 3rd in a Blitzball Tournament.
After you do this you'll want to head to Luca and speak with the bartender. As seen in the screenshot below, this is who gives you the World Champion.
After you acquire the World Champion you'll need to acquire the Jupiter Crest and the Jupiter Sigil to fully upgrade your new weapon.
Acquiring the Jupiter Crest is easy as can be. After you unlock Blitzball and complete the story events that take place in Luca you'll be able to find the Jupiter Crest inside the Besaid Auroch's locker room. It's in the back of the room located in one of the lockers. Seen in my screenshot below.
The Jupiter Sigil is arguably one of the most annoying Sigils in the game to obtain. In order to obtain this you'll need to play Blitzball... Lots of it. I spent about 30-45hrs of played time learning Blitzball and acquiring this Sigil but I honestly believe you could do it in less. Most of my time was spent researching for this guide.
In order for this item to appear on the list of prizes for a league you'll first need to acquire Attack Reels, Status Reels and the Auroch Reels. All of these are Overdrives for Wakka and they're rewards from Tournaments and Leagues in Blitzball. Most guides on the internet claim you need to get all three of these before the Jupiter Sigil appear as a reward. I for some reason remember seeing Jupiter Sigil appear before I got any of the reels but maybe I was tripping.
You can alter the prizes that are available in Tournaments and Leagues by planning ahead and also doing soft game resets. Tournaments don't require a game reset all you have to do to change their prizes is keep selecting "Play Blitzball" at a save sphere and then exiting until different prizes are shown. It will take 5 - 7 times usually for this to happen. First the tournament will end and appear in gray, like in the screenshot above, then it'll change prizes.
Leagues are a bit trickier to alter the prizes of. You can't change the prize of a League if you've already started one so you'll have to finish any leagues before you do this trick. After I beat the final game in the League when it shows the screen that says 'Continue Playing Blitzball' or 'Back' you'll want to use a save state and then select 'Back'. Select the Save Sphere and 'Play Blitzball' - if the prize isn't a Jupiter Sigil then reload the game and try again.
You can also do this trick if you don't have save states. Select 'Back' just like you would in the list of steps above but instead of selecting 'Play Blitzball' save your game then select play Blitzball. If it doesn't have the rewards you want you can reset your game by holding R1, R2, L1, L2 Start and Select.