First, shoot the lock open to pass through the door. As the van drives through the passage, clear out the mines before they touch the van. Press the to aim, and to shoot.
Once you get out into the open, you will be attacked by swarms of Robo-Falcons. Clear them out before any of them come hurling your way. Then, as you press on, Clockwerk's Death Ray will zap a nearby mountain, causing boulders to roll down towards you. Blast them to avoid serious damage.
After that, you will be attacked again by more Robo-Falcons. There are also some more mines you will need to destroy. As you venture down the path to the cavern, there are more mines, and the Death Ray will strike again. As the rocks roll towards you, blast through them as quickly as possible before they make contact with the van. After that, you will be attacked by one last wave of Robo-Falcons. After they are cleared, the mission is complete.