if you get the door to be higher leveled than the boss, its almost indestructible. the strat i found to be most effective is to focus upgrading the bed for more money. only upgrading door and gun when its required for the bed
Yes okay, I was immediately able to beat the game. My only problem was that I didn't know that you could upgrade the bed.
EDIT : And actually because of that, the comment you replied to makes no sense now, because winning is too easy. But the main gist still stands, there's some rebalancing to be done.