Despite the name, there was no real boss battle, It did looked nice, but I experience lots of visual Glitches, also the culling of the walls was acting weird, The "Boss" is super dumb and it doesn't really do much.
If it was a real challenge, the Idea of gathering things to defeat the boss would be a good one, Remove the pistol and imagine that instead of gathering keys, you gather parts of a gun or parts of a bomb, and you need to escape the bosses attack while you do so, after that you need to find time to build that gun and attack the boss.
PD: I like to think that this is not an FPS but a 3rd person shooter in which the playable character is a giant pistol