When you chase Bumblebee as Starscream, follow him until he transforms. To kill Bumblebee, jump around him in circles while shooting both your guns (light and heavy). After he dies, Starscream will have the AllSpark and Barricade will attack. There are two ways to kill him: the first is to move left and right while shooting your heavy weapon. When he (Barricade) transforms, jump off the building and destroy trees to restore your health. Climb back up and repeat the process. The other way is to transform and fly back and forth while shooting.