Here is my idea for handling the Spider Mastermind.
The way I see it, the way to effectively inspire the same amount of sheer terror that it inspired in the original Doom is to make it fire 12 shots of 1d6. Yes, that means it can effectively, if lucky, kill you in one shot. I don't think that's unbalanced, really, because it's supposed to be slightly less dominating than the Cyberdemon, or, depending on your outlook, more dominating.
Obviously, it'd have unlimited ammo.
To allow for victory, it'd be knocked back very easily, to simulate how easy it flinched in the original game.
This is just my kinda brainstorming ideas.
I'm sorry if this is already slated for release.