Xatrom Command

This is the second boss. It’s a slow moving tank-looking alien which moves towards you while occasionally firing a horizontal line of balls/missels at you. Just keep firing at him and dodging the shots until it gets too close, then move to the other side of the map and repeat until its health runs out (the bar at the bottom of the screen will show you its health). Once the boss’s health runs out, the achievement will pop.