When battling Dr. Krone's sentinel use the Pulse Gun to destroy one of his guns. He will retaliate by firing a barrage of artillery at you which you can escape by slowing down time and going to the far right corner. Then face the wall so you are looking up at the sentinel and then jump up onto the platform and run to the right corner. This can be done to avoid all three of his barrages.
Unlock Dr. Aiden Krone as a Playable Character in Multiplayer
Added 20 Feb 2008, ID #439
Go to Multiplayer at the 'Options' menu and select 'Model'. Then press 'Left' once on the directional pad so Krone is highlighted and press Y. Then type RXYMCPENCJ and Krone will become a playable character.