Half of Life for 5 Seconds:
Pause, then press Up, Up, Down, Down, Left, Right, Left, Right, X, Circle.
5 Seconds for Half of Life:
Pause, then press Circle, X, Right, Left, Right, Left, Down, Down, Up, Up.
Disable Enemies Markers:
At the mode selection screen press Right, Right, Right, Right, Left, Down, Up, Right.