Play the rooftop level just before the intermission where Max blows up the satellite tower on the normal difficulty setting. At the beginning of the level, don't collect the items against the wall and don't sneak up on the first man, because he has to see you. Walk behind him when he stops, wait until he turns around and then press Y to flip him. This will make you fall underneath the floor and pass all the guards. Run to the end of the level where the final locked gate is located. Run into it to complete the level without running out of time or into any guards.
Complete the game and wait for the credits to end. An extended sequence featuring the game's true ending will start.