Heres how to defeat Captain Slag. Take out any weapon and start shooting him. Always keep your distance. Once you inflict enough damage he'll run away and sent out his crew members to kill you. Destroy them. Then Captain Slag will come back just keep shooting him. Later in the fight he'll shoot these streams of fire along the ground. Just jump over them. When he shoots out these blue boomerangs, shoot those to destroy them. If you need more help you can contact me.