Enter the options menu, select "Multiplayer", "Model", then press Left to highlight Dr. Aiden Krone. Press Triangle, then enter "RXYMCPENCJ" as a code to unlock the Dr. Aiden Krone skin in Multiplayer mode. -From: vitruvian_soulless
While playing in Single Player mode, select "System Link" at the main menu, then "Host" a game. Select any game type. Set the gravity to "Very Low", jumping to "Very High", and movement to "Very Fast". Host the game, and it will fail. Return to the main menu, then select Single Player mode. The settings from system link will continue into your single player game.
In the battle 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 look 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.