Before you get to the Bosses, fill up your adrenaline meter. This can be easily done by shooting head shots. Once your meter is full, save it until you get to the Boss. Once you see him, press Up. You will now have limited immortality and unlimited ammunition. If you have a sniper rifle, you can shoot the Boss in the head a few times. If you have a sub-machine gun such as a Thompson or an STG-44, you can run up to the Nemesis and shoot him. Another way to inflict serious damage is to use a rocket launcher. Note that you will not have unlimited ammunition for this -- it is best to have a good back-up weapon. If you still have not killed him and your meter is almost empty, retreat to some cover. Note: If there are enemies surrounding the Boss, which is true in most circumstances, eliminate them before you attack the Boss or you will be slaughtered once your meter is empty.