Enter the options menu then select multi-player and then model type press back to highlight the hidden skin then press triangle and enter this code RXYMCPENCJ and this will unlock the skin for multi-player mode.

If you kill someone or step in oil or mud, walk on cement or somewhere clean and look at your footprint. It will be in great detail and read "Atrox" at the top/middle of your shoe print. "Atrox" could be a reference to a type of snake, a band, and a television show or movie.

In the fight with Dr. Krone's gigantic Sentinel, when you destroy one of his guns with the Pulse Gun, Krone will send artillery raining on your roof. In order to escape this barrage, slow time and go into the far right corner. Face the wall and up at Krone's Sentinel. Jump up on the platform. Keep running to the right into the corner. You will take no damage while in that corner. This works for all three barrages.