After getting the submarine and completing the Underwater Reactor, sail to a peninsula west of Costa del Sol and look for a river. Submerge the sub, and you will find a cave. Go through it and after you surface, land nearby and enter a grotto behind the waterfall. With Vincent in the party, watch the cutscenes between him and Lucrecia. After returning to the world map, fight about 10 battles and return to the cave. The last weapon and the Limit Break Chaos will be in front of the crystal.