I have been waiting for an update and it has finally arrived, I have only managed to complete Hard mode as of now, but may be able to tackle the rest of the challenges. I shall see.
I start understanding that there isn't much the player can do in nightmare mode as the best you can do is put a lock on the door and hope he doesn't bang on it immediately, i've had scenarios where i was running between doors for 20 minutes because he wouldn't give me any time to work on opening my door.
Wasting a lot of resources avoiding damage between doors just to see if he has spawned in a door feels frustrating. It could use some sort of grace period where he can't bang on the door or come out of it after the lock is placed, or he needs to make a noise when he spawns to let you know he is in play.
Otherwise i'm afraid 3 doors is way too much work for little reward for an enemy that has no tell to its arrival.
Just to add to this, maybe it could be a bug as to why he is banging on the lock so frequently? Because even though he isn't right about to leave the door he'd bang on the lock immediately as i place it which makes me believe there is an issue with his pacing through the room.