Alien Breed

There are bosses on several levels throughout the game and you'll need to defeat one with any weapon other than the Assault Rifle. Your first opportunity will be in Alien Breed Level 3 - Power System. You can switch between weapons as long as you never use the Assault Rifle. The bosses are larger than regular enemies but still have to touch you to damage you. Keep your distance, keep firing and you'll take the boss quickly.