In the level where you chase Bumblebee as Starscraem, follow him until he transforms. To kill Bumblebee, jump around him in circles while shooting both guns (light and heavy). When he dies, Starscreem will have the Alspark 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 Barricade transforms, jump off the building and destroy trees. This will restore your health. Climb back up and repeat the process. The other way is to transform and fly back and forward while shooting.