The Death Ray was a powerful weapon invented by Lash of Black Hole. The Death Ray fired a giant beam of energy in a 3-space column down the screen, which reduced full health units to 2HP and others to 1HP. Its weak point was the entrance, right in front of the cannon. Only one Death Ray was built, and it saw fairly limited service in the climax of the Macro Land wars in Final Front.
Sturm can only fire the Death Ray on Days 7, 14, 21 and 28, but the weapon cannot damage his own troops.
Don't keep your troops in the Death Ray's firing range on the aforementioned days! Scatter them out so they'll be able to avoid the Death Ray, then bring them back together to counterattack. Colin is one of the best choices for the first CO, which will be right in the middle.