To beat Sekto,you should be carrying a full ammo clip for each type of ammo that causes heavy damage. First, you need to run around and wait till a part of the roof falls downand when it does run behind it.

From the side shoot the 2 launchers at both sides of his machine with heavy ammo (if you run out there are some crates behind youto fill up your ammo). Repeat these steps until the lights at both sides of the machine go out.