Briefing: "Our brothers within GDI tell of us of A-10 strike jets scheduled to be deployed here soon. Our suppliers have delivered new Surface to Air Missiles to aid you. Use the SAMs to defend your base, then seek out their base and destroy it."
Walkthrough: It's strange that Seth talked about Lagos but according to map you'll fight in Mauritania. Aside from common destroying of all GDI units and structures you also need build a SAM-Site. So don't sell your CY before you do so.
You can rush this mission or you can just camp and build sizeable force before you attack GDI base.