final attacks only last for the same amount of times as the level of your final attack materia AND the materia joined to it, therefore only have a maximum of 5 times, (any magic or summon materia including restore will work, however most people use phoenix as it gives an extra "5 lives"; when master summon or master materia is attached i think the strongest one is used, therefore KOTR and ultima, i dont think any other magic is cast)...

when getting final attack, you need to...at least have a persons ultimate weapon - i recommend Cloud or Barret; both of their 3rd limits and a ribbon... also helpful if you have an enemy skill with big guard, magic hammer, white wind, etc and 4x cut, materia which gives you haste, hp plus, mp plus, good armour, and lots of items (as in more than 50) such as maiden's kiss, (thing to cure mini), antidote, (thing to cure stone/petrify, think its called soft) and/or have a lot of remedies, X-potion or elixer, etc... (you can do without the following items but it takes longer and is much harder), AND A LOT OF NORMAL GP...

go into the battle arena, get enough GP to buy Omnislash and W-summon (remember not to leave the battle arena area or you will lose all your battle GP and hopefully you dont get a POWER CUT), then talk to the lady at the counter (Battle Square) and she will talk about a special battle in which you can enter (only Cloud)...when you win this battle you will get Final Attack as a prize...You can only fight this battle after obtaining BOTH OMNISLASH AND W-SUMMON...

And you can keep on fighting in the special arena though you will not get another final attack that way...