To get the final attack materia you must first buy the w-summon materia and the omnislash from the battle arena. then you must get Clouds ultima weapon, after you have all of this go and talk to the woman and she willl ask you if you want to compete in speacial battles were there will be harder monsters, and the final monster will be the proud clod! you can only do this with Cloud and his ultima weapon, so lvl up and make sure to equip a ribbon!