Turn on all the valves you find. If any type of guard stands near a valve that is on, they will pass out.
To defeat riot guards, shoot a shotgun a few times at the black panel on their backs.
Use the minigun to destroy the robot guards in the final battle. Try to line the robots up for better results. The force of the gun will push the robots. If they are in line with each another, they will both become helpless.