Head to the marker, but when you enter a battles you’ll fight alone. After you take some damage your party members will come to your aid. You don’t have to fight the enemies, but they give away a good amount of AP. Walk towards the marker after the battle and you’ll find Wakka. In the next area just head to the right to find Kimahri, then go around and you’ll eventually reach Rikku.
During the battle have Rikku steal from the machinas, they’ll fall apart soon afterwards. Continue on around and in the next area, follow the marker. There’s a Sandragora guarding a treasure chest(you’ll see it in a sandtrap), it’s quite tough to beat though. It does you in by turning your party members against each other…so make sure you have some remedies available. Use Fira and regular attacks against it. You’ll have to fight it again to get through.
An Al Bhed Primer is located within the desert to the left in the shadows of ruins.