Find: Ultima Materia & Catastrophe Limit Break (Barret).

Fly to Corel. Enter. Talk to everyone and you gather that Shinra is trying to get a Huge Materia from the reactor. Head there by going up to the top under the sign marked North Corel. Cross the bridge and follow the tracks back to the reactor. Fight the guards here and after you finish, a train will leave with the Huge Materia.

You climb in an engine and follow them. After Cid says to do hit the alternating levers, just keep hitting ARROW-UP and TRIANGLE at the same time. Do this really really fast so you can catch up to the train. Get on and get to the front car in ten minutes or less. As you jump to each car, you have to fight. Run from battle if necessary.

Fight the guard at the front car and then stop the train. You have to hit and at the same time, then and . Keep doing this while your friends are yapping and save the town. If you do, you get the Huge Materia, and also the little boy to the left gives you the Ultima Materia. You also get a free rest at the Inn. Talk to the lady in the house down and left from the Inn to get Barret's Level 4 Limit Break Manual, Catastrophe.

If you don't save the town, you don't get the Huge Materia, you have to pay the boy 50,000 gil for the Ultima Materia, and you don't get free room and board at the Inn. So, save the town. If you fail, reset and try again.