You dont need the shotgun for the boss. It does help tho. I cant remember where, but there is a broken shotgun in the house that you can swop for the working one. For the boss, he has 2 stages. When you drop in just blat blat him with pistol/shotgun rounds for a bit, and hell collapse, before going and grabbing himself a super weapon. There is a chainsaw you can pick up in the same room he gets his super weapon. Use this to kill him. Anung pointed out a very useful strategy is yo can actually crouch to avoid his big attacks and then you get free chainsaw attacks on his feet. Its not a very good boss fight and doesn't give you much feedback that you're actually pursuing the correct strategy, but rest assured as long as you are hitting him with the chainsaw/gun, he will die. Theres no trick to beating him, but it does feel that way at first.