Overview
you can glich the game after you read the guide!
first step
first of all, get to the final boss (witch is the only boss in the game)
second step
second step is to enable cheats then tyope in sv_cheats 1 after doing that enable noclip.
final step
fly though the floor. you will see the moon go to it and the game will telleport you back but insted of continuing with the fight you will be at the end of the fight. complete the game as usally and then you are done.