The Onion Knight
You must obtain the Celestial Mirror before you can obtain or create any of the Celestial Weapons. Click on the link for more information on how to obtain the Cloud Mirror and Celestial Mirror.
The Onion Knight is Lulu’s Celestial Weapon. It needs to be combined with the Venus Crest and the Venus Sigil in order to add on its special abilities. It is considered to be one of the most difficult Celestial Weapons to fully power up due to the difficulty of obtaining the Venus Sigil and completing the lightning dodger challenge.
You can obtain the Onion Knight after you have defeated Geosgaeno near Baaj Temple. A treasure chest will appear in the southern most portion of the water. Check out the Baaj Temple side quest section for more information on how to defeat Geosgaeno.
The Venus Crest can be found in Guadosalam in the Farplane. It can be obtained as soon as Maester Seymour leaves Guadosalam during a regular play through. Travel back to Guadosalam and enter the Farplane and you will see a treasure chest on the left side of the screen. Open it to receive the Venus Crest.
In order to obtain the Venus Sigil you must travel to the Thunder Plains and dodge 200 lightning bolt strikes in a row. Make sure that you have the [No Encounters] ability on your equipment.
Lightning Dodger Side Quest:
There is nothing on the screen to assist you with keeping track of how many lightning bolts you have dodged so it is advisable that you over estimate just to be sure. The prizes for dodging lightning bolts, including the Venus Sigil, appear in the treasure chest outside of Rin’s Travel Agency.
There is a secret area you can go to near the entrance of the Thunder Plains that has a huge crater just off to the left side of the screen. The lightning bolts will strike each time you enter the crater making the process of dodging them much more predictable.
Check out the video below for more information:
This challenge is considered one of the most difficult in the game. Good luck!
Return to Macalania Woods to the area where you received the Celestial Mirror once you have obtained both the Venus Crest and the Venus Sigil. Offer the Onion Knight up twice to fully power it up using both the Crest and Sigil.
The Onion Knight has the following abilities when fully powered up:
Break Damage Limit - Allows Lulu to do more than 9,999 damage in a single attack
Triple Overdrive - Charges Lulu’s Overdrive at triple the speed
Magic Booster - Uses more MP to heighten the power of Lulu’s spells
One MP Cost - Causes all of Lulu’s spells to cost 1 MP (which negates the downside of the Magic Booster ability)