You must be at the point where you can enter Gemity freely (usually on a cleared-continued game). Go to the arena and clear the S-Rank single battle to win the Imperial Scroll (Attack +1000)

Go to a workshop and via Fayt's Writing talent. Select 'Sepcify Plan' and choose the Imperial Scroll and refine it for one time lap. Now you should have two 1000 point attacks on the scroll. Repeat the refinement for more attacks.

Synthesize the Imperial Scroll on someone's second best weapon and it should let you unleash a fairly powerful 9999 attack.