The following Bosses in Sector 2 have the corresponding weak spots:
Colossus (Base Boss) Climb the stairs until at the very top and look down at Colossus's head. When the head opens, shoot it with a shotgun for best results (1/5 to 1/4 damage).
Enforcer (City Boss) On the machine's back near the bottom, is a circular lighted area. Your crosshairs will lock on to that area. Shoot it with a shotgun or rocket launcher for best results.
Mantis (Ruins Boss) Get close to the bug and shoot the underside. Shooting its shell will not do anything, as it deflects most of the shots. The best weapon is the Photon MG because it stops Mantis in its tracks.