How do you kill Starscream

Get Galvatron to blast him again like he did in the original movie.
You use a powerful gun say use Optimus Prime

Actually, you don't. Not even Galvatron ever killed Starscream. He came back - so many times, they finally gave up on getting rid of him and left him drifting in space in G1, last I heard. The Maximals in Beast Wars even tried to copy his "indestructable spark" and create a warrior for good. Needless to say, it didn't work out.