Use the Aero DP then climb up to Selphie. Choose 2 teams: Squall’s team will go to Balamb Garden to warn of the impending attack while Selphie’s team will infiltrate the Missile Base to try prevent the launch. Note that Squall’s scenario is harder, but provides a great opportunity to gain some levels.

When Squall’s team reaches the Galbadia Desert Station, sneak onto the train to your left. Have Selphie’s team return to Deling City to restock supplies and remodel weapons before going to the Missile Base. When finished there, get back in the car and travel northwest of D-District Prison to reach the base. Drive inside.