Starscream always choose the easiest way to do things, not the right one. You should have electrocuted him. You can still do it. Or melt him! [Wouldn't that be fun?]

I did, some time ago. He's not here. or anywhere, not anymore. [All he has now are memories but those are good ones.]

Unicron doesn't have a physical form but we can still fight him, hence why we get powers. The FirstForged can give us boons like more resistance, strength or the ability to make really fast calculations.