In the vet's office on level 13, walk into the small office on the far right side. Approach the computer and press ACTION to play Missile Command. Then, complete the level to unlock the game at the "Atari Games" menu.
This Boss battle is your mission in level 5. Press the buttons as instructed by the bottom captions. Do this at random so that the Infiltrator cannot predict your attack. When his health is down by 24, he will get a limp leg which will slow him down.