This mission is basically a battle between Optimus Prime and Brawl. When the battle starts, just strafe while locked on and shoot. If you run out of energy, just keep tapping so you stay locked on. Remember, he can't shoot you if you strafe. Eventually Brawl will say, "I'm a tank, you're a truck -- how do you expect to beat me?". When he says this, he will transform into a tank, and human tanks will attack you. Pick up one of the tanks and press near Brawl to hit him with it. Do this about five times and he's dead.