Spirit Lance is a Celestial Weapon and it is the most powerful weapon for Kimahri 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 Spirit Lance you will need to obtain the Spirit Lance weapon itself first, then use the 
Saturn Crest and the Saturn Sigil to power it up.
Start off by traveling to the Thunder Plains. There are Qactuar Stones scattered all throughout the plains that resemble a tombstone with a picture of a Cactuar on it. You need to find 3 stones and pray in front of them by pressing Square near them while they are glowing.
Before you start, make sure that you read the book in Rin’s Travel Agency in the middle of the Thunder Plains.
The stones each take turns glowing so wander around until you find one that is lit up - you may end up waiting quite a while if you stick it out near one. Start off near the beginning of the Thunder Plains and work your way back.
There are 2 stones in the first area and 6 more in the second. Making sure that you have the 
No Encounters ability can make this task much more manageable.
After you pray at the third stone a small Qactuar ghost will appear. Follow the ghost along until it reaches an old tower that is leaning over on the right side of the screen (in the first area). Press Square to pray in this area and a treasure chest will appear.
Open the chest using the Celestial Mirror to receive the Spirit Lance.
The Saturn Crest is located on Mount Gagazet. Double check your inventory to make sure that you have not already picked up. If not, it can be found in the passageway leading through to Zanarkand just after the area where you fought Seymour Flux. Use the screenshots below to help guide you to the area.
In order to obtain the Saturn Sigil you need to complete the Macalania Woods Butterfly Hunt. Travel to Macalania Woods and use the guide below for assistance:
Return to Macalania Woods to the area where you received the Celestial Mirror once you have obtained both the Saturn Crest and the Saturn Sigil. Offer World Champion up twice to fully power it up using both the Crest and Sigil.
Spirit Lance has the following abilities when fully powered up:
Break Damage Limit - Allows Kimahri to do more than 9,999 damage in a single attack
Triple Overdrive - Charges Kimahri’s Overdrive at triple the speed
Double AP - Allows Kimahri to earn double the AP for each battle
Evade & Counter - Allows Kimahri 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.
