Easily the most difficult level I've beaten.

In the opening I grab shotgun, kill 2 shotgunners then move against the mancubus to open secret door to the blue sphere.

I grab armour and continue to kill all groups of enemies. I go to the central area to fight whenever I can to make things easier.

When I lower the red key, I sprint to grab it then camp by the red door. I really take my time here and kill almost everything.