When at the start of the final Boss level, quickly activate your jetpack and fly straight up as high as possible. You will see a blimp that has "Duf beer" written on its side. Shoot the blimp with your RPG or Devastator and a large amount of items that include medipacks and ammunition will fall out.
If you have a jet pack, fly up and destroy the beer blimp. Guns and health will scatter on the ground, allowing the final Boss to be killed easier. Use the Devastater for best results.
There is a tenth level on Shrapnel City called Tier Drops. To access it, go to the movie level and go to the stage with the NASA sign and the body parts with a telescope. Kill the alien with the chain gun and press [Use] on the sign. A door should appear with the caution sign that ends the level. Do not go to the stage with the ship, or this will not work.
To access a secret Freeway level in Shrapnel City, go to the Hotel Hell level. At the fountain by the pool, step through the waterfall and go behind the bush. You will see a gray radioactive waste sign. This leads to the secret Freeway level.
There are two tripwires inside the room with the Boss; one closes the door and the other brings the Boss. When you walk in, turn around so that you are facing the door. When the door starts to close, use steroids to run past it. You can now open the door and it will not close. Trigger the Boss and run behind the shelves for cover. Duck down and shoot his head. He will not run away.