Play through the eighth level until you get to the bomb in the kryptonite room. Freeze it with your freezing breath and when you walk out Jimmy Olsen should be waiting for you. Talk to him then when he leaves go out through the path where the kryptonite room is. Go all the way around. When you get back in the same place you should see Darkseid waiting for you. Stand behind him (at a safe distance) and toast him with your heat vision. He should just stand there as you kill him. Pick him up and take him to the cops at the beginning of the level.
Become a Car
To become a car, first go to practice in the title screen, then while in the practice mode choose if you would like to kill the enemy shooting at you or not it doesn't matter, then pick up the car (with B) and fly with it (Z) then go through all the rings around the Lexcorp building and once you fly through the last red ring (remember you MUST fly through it, if you walk through it the trick won't work) the game should glitch, and you should end up in the car.
Becoming a car is pretty cool, but what if you want to want to get out? Without restarting the mission there is a way. To get out, fly at full speed in the car at one of the poles right where you got the car, and as you hit it pull up abruptly untill you fly over, and the car is no longer with you!!! This saves a lot of time. It may take 1 or 2 tries but it works!
While playing Superman, let go of the analog controller. Then use the d-pad to move the camera around! Pressing up makes the camera move in, pressing down makes the camera move out, pressing left makes it go left, and pressing right makes it go right. If you move the camera out i find that the controlls to the game dont feel so slow!
Stop the Bombs
To stop the bombs at the dam and the Daily Planet, use your freeze breath. Make sure they're frozen solid.