Finally, after 5 months, I beat the final fat shotgun boss. However I had to break the rules a bit. In the T-Shaped Area where there are two guards ahead, I shot them with 8 bullets in my rifle, and then somehow dragged their guns to the boss fight through the door by switching and moving constantly. Then I pressed Q and spammed my rifle, then took the pistol on the ground, which I guess is illegal, but it helped me win the fight :)
It took me 5 months xD