In Chapter 3, go to the gorge in the Calm Lands, and do the Rescue Tourists Sidequest. One of the tourists (the guy running past you towards the entrance) will have the Besaid Key. This key allows you to open the chest in Besaid Temple and get a Search Sphere. Also, when you see the two children run to the right, go to the left. Examine the small area and you will find a tourist hiding unseen. You need to rescue him to be able to warp to the first Hidden Chamber. And when you're fighting Yojimbo, have a Mega-Potion ready to use. His Zanmato attack will drop your entire party's HP and MP to 1. Have Yuna or Rikku in Gunner. Trigger Happy LV3 will buy you some time for the others to take action.