Yeah, this old routine again. Stats first.
Spider Mastermind:
A (the "dark yellow" color used by Pain Elementals)
Health: 170+10*difficulty
Armor: 3
Speed: 110%
Melee Damage: Avoids melee, 1d3+13 if forced
Accuracy: +8
Ranged Damage: (1d8)x6/9-48, armor applied to each shot individually, armor effect halved
Description: A larger Arachnotron with a larger gun, you hope this is the worst Hell has to offer.
The idea is that it's a bit more fragile than the Cyberdemon (lower armor and health), but its attacks are far harder to avoid - particularly as Dodgemaster does not render you largely invincible like it would against Cybie.
Now, the obvious place for this would be after the first wave of Arachnotrons in the Spider's Lair. But I thought about the chain of events connecting a couple special levels... why not replace the Cyberdemon with the Spider Mastermind if you clear the Spider's Lair?