There is just one "concealed" enemy in Mission 01. Right away turn around to find a red mantis securing a Blue Orb piece when you drop down the damaged bridge (after the cutscene midway through the mission). Make certain to kill it. The remainder of the enemies remain in plain sight. The mission is just one long bridge with enemies on it-- you can just enter one direction and the enemies are easy to see. Just kill all of them to get the "Protect The People" achievement.