Quote Originally Posted by Omecle View Post
The Final Attack materia can only be gained by defeating the special battle in the Battle Arena. This special battle can only be obtained once you receive the Omnislash limit break and W-Summon materia.
Nope, you can also win it as a chocobo racing gift, as TifaLockhart7 mentioned, but the special battle is far quicker, easier, and less frustrating. The chocobo racing is a matter of sheer luck.