Tutorial: How to Get the Seitengrat Bow (100% guaranteed)

Get the invisible chest to spawn and open it at the exact moment.

Relying on luck gives you one percent chance of the invisible chest spawning, and one percent chance of it containing the Seitengrat Bow (one of the strongest weapons in FF12). With this method, you will spawn it 100% of the times, and correctly time opening the chest to get the bow 100% of the time.

- Get the Diamond Armlet from the very first stage in Trial Mode. Get the diamond Armlet, finish that stage and the game auto-saves. Now continue playing until you get access to the Aerodrome in Rabanastre..
- You can repeat this method on the same trip from Rabanastre to Nalbina.
- To err on the safe side, do this on your second (or third, or fourth, or...) taking an Airship. The first time you will view an introductory cut scene and that might screw up some of the factors at play.

Steps show in the video:
1) Play the beginning part of the game with Reks until you get to the save crystal. In the next area Reks defeats three soldiers. Go back to the save crystal and save your game on a separate save file. You can continue playing the game, but save your progress on another save file so you can always reload the save file with Reks.
2) On your main file: Go to Rabanastre Westgate (with the Diamond Armlet equipped) and enter the Aerodrome.
3) Take the Airship from Rabanastre to Nalbina
4) Go to the Air Deck with only Vaan (with the Diamond Armlet equipped) in your active party. (Guest characters are fine.) The game creates an auto-save.
5) Close application by holding down the PS button on your controller
6) Start the game again.
7) Load the save file in Nalbina Fortress / Upper Apartments (with Reks
8) Cast Cure on Reks until you find the number sequence of 88, 97, 90. (Stay in the area of the save crystal or you will lose your auto-save on the Airship!!)
9) After getting the 88, 97, 90 sequence, pause game by pressing the Options-button, quit to title, load thee auto-save on the Airship
10) Look at the Boonswaggled Bookkeeper: he has to walk up and down the steps three times.
11) The fourth time coming up, open the invisible chest when the Bookeeper steps at the top of the steps on the upper deck. This gives you the Seitengrat Bow.
12) Leave and re-enter the Air Deck tp update your auto-save
13) Repeat from step 5 onwards to do this again.

Hopefully Final Fantasy XII The Zodiac Age will still be enjoyable without any difficulty. You'll defeat every enemy that stands in your way!

As I've also said in his comments, thanks to LetsGetPitted for showing this method.