Approaching the end of Disc 2, you'll be able to fight one of the undefeated WEAPONS still roaming the overworld. The Ultimate Weapon will be located flying over east of Junon. However, if you fail to defeat it and you lose, but you still manage to survive or it plains flees from the battle (which is what it does all of the time), the Ultimate Weapon will be flying over Lucrecia's Cave. If you haven't beaten Ultimate WEAPON before Disc 2, it'll be available during Disc 3 as well.

It's not very difficult to defeat Ultimate WEAPON and its basically an easy strategy to do it. Just equip some strong Materia like Quadra Magic and strong summons like Bahamut ZERO. Put up some strong enhancements for your defense and magic defense. Big Guard, for example can be great for resisting against high-damaging attacks. Just equip the Long Range Materia and then keep up damage against Ultimate WEAPON. Upon defeat of Ultimate WEAPON gives you Cloud's ultimate weapon, Ultima Weapon, it also opens up the path to the Anicent Forest.