From what I remember you really just need a few weapons, runes, and preator upgrades to focus on before the cyber demon and it gets "easier"
The runes would be:
Rich get Richer
Armored offensive
In flight mobility or Vaccum (whichever is more handy at the time)
As for the weapons:
BFG
Gauss cannon - Siege mode (fully upgraded)
Chaingun - Mobile Turret (fully upgraded)
As for the preator suit? Upgrade the powerup buffs to stave off dying.
Basically run around, stun as many imps as possible for Glory kills to keep your armor up and make sure that you keep the armor max high with the Ardent upgrades.
That way you have infinite ammo just before the boss fights. Then it's just a matter of timing with the Gauss or Chaingun (depending on what you prefer) to make the fight go faster.
Edit: Also Holograms are your friends