At the start of the map you are facing the problem that a Cyberdemon is standing right behind you, while a large horde of Mancubi and Chaingunners is waiting in front of you. Especially at higher skill levels it might prove useful to avoid alerting the Cyberdemon and instead cause infighting to ensue between the Cyberdemon and the other enemies, as there is only a small number of powerups to help you survive the fight otherwise. Even though that might cost some health points (mainly due to the overwhelming number of hitscan enemies you have to face), the Cyberdemon will be able to clear a huge part of the area that lies before you, while at the same time it will take a considerable amount of damage itself, weakening it.
From the starting point, take the right route (i.e. the one eventually leading to a large hexagonal opening in the wall with a Mancubus standing inside it). Follow it until you reach the first wooden door there, open it and walk up the stairs. There you should see a pool, with a pillar-like structure the middle. Now to your left there is a wall with a demonic face on it and a small candle in front of it: Hit it with your use/open key (default: space); that will cause the aforementioned pillar to lower (Reward: Plasma Rifle)