For infinite health, die at the same time you kill a Boss. This is best done when fighting Revenent Prime. Get him down to very low health and wait until he starts to destroy the building you are standing on, then let him have it. As you fall to your death the game will continue to show a short cinematic sequence, and you will start the next level with 0 health. Enemies cannot kill you, but you can still die if you fall off the level. Note: The health warning will constantly sound.