In order to get Yuffie's final limit break, you need to unlock the six prior limit breaks and obtain the item that teaches Yuffie her final limit break. The way to unlock each limit break differs for each character but are essentially similar. You unlock the second limit of each stage by using the first limit of that stage a bunch of times, and you unlock the next stage by getting a certain number of kills.
Yuffie's limit breaks:
Stage 1: Greased Lightning, Clear Tranquil
Stage 2: Landscaper, Bloodfest
Stage 3: Gauntlet, Doom of the Living
Stage 4: All Creation
Unlock order:
Use Greased Lightning 8 times in order to unlock Clear Tranquil.
Get 64 kills with Yuffie in order to unlock Landscaper.
Use Landscaper 7 times in order to unlock Bloodfest.
Get 64 kills with Yuffie after unlocking Landscaper in order to unlock Gauntlet.
Use Gauntlet 6 times in order to unlock Doom of the Living.
Use the All Creation item after unlocking the six previous limit breaks in order to unlock All Creation.
The All Creation item is not hard to acquire. You just need to head to Wutai and beat the boss Godo in the optional Pagoda sidequest (although you will have to go through the whole business of losing your materia and rescuing Yuffie from Don Corneo first).