The Cyberdemon fight sucks so here’s how to make it suck less.
Before you take on the Cyberdemon, make sure you have an upgraded Super Shotgun preferably with the Double Trouble (Two shots before reload) mastery mod. Also have at least 2 shots in your BFG.
It would also be nice to have a lock-on rocket launcher and/or fully upgraded Precision Bolt Gauss Cannon.
Boss Fight – Phase 1
The Cyberdemon will launch you to the center of the arena when the fight starts. Once you can move again, keep moving. If you stand still too long then his rocket launcher will chew through your health. At first, just hop around in circles while pummeling the Cyberdemon with rockets until the second part of phase 1 starts. You can tell it started by a small change in attack pattern along with a carpet bombing of rockets. this is when you shoot ONE BFG shot to stun him then get in close with the Super Shotgun. This is where Double Trouble and Fast Reload mods come in.
Repeatedly shoot the Cyberdemon’s head until he dies. If he uses a jetpack to dash across the arena, hit him with the lock on rocket launcher to make him falter then continue filling his cranium with pellets. Eventually the Cyberdemon’s leg will flash a Glory Kill indicator. Finish him off to end Phase 1 of the fight. Along the edge of the arena is a BFG pickup. Get that if possible.
Boss Fight – Phase 2
After you rupture the Argent Accumulator, you will be transported to Hell along with the (formerly) dead Cyberdemon. Tendrils of energy will strike him and resurrect him. Then he will summon walls on either side. Shoot your BFG at him to stun him then begin shooting him with the Super Shotgun again. There will be no need to worry about ammo or health because the Super Shotgun causes it to rain off the Cyberdemon’s body.
After 10 or so shots, back up a little so the Cyberdemon can do his wall attack. while you are backing away, hit him with a few lock on rockets. The Cyberdemon should either start slashing fire arcs at you or do his wall attack. If he does the wall attack, shoot him with the BFG and close in with the Super Shotgun again. That will be enough to trigger a Glory Kill to finish the fight. If the Cyberdemon does his flame arc attack, jump over him and hit him with a few rockets or a Precision Bolt.
Thats about it. I used this method with fully upgraded health and ammo on normal difficulty so no gurantees for higher difficulties or Argent loadouts.