As can be seen in the video, if you aim facing away from the door and back in to it / turn the door unloads and you can enter the boss room early. If you then go behind the pillar and continue to the left of the seats, the Death Angels then spawn and you can start the boss battle.

Before you start this battle, you'll need to get the Smoke Grenades and capture all the Poltergeists (except the one in the Library area) to get the Astral Ability, or else you can't complete this boss battle or other areas later in the game (Junkyard, HMS Victoria, etc.)