What's a good strategy for defeating the Cyberdemon?

How funny the previous answer may be, surprisingly it holds -- DoomRL has its own reimplementation of strafing - the dodge mechanics. Basically, if when the enemy fires, your last action was a move not directly towards or backwards from the enemy, there is a chance that the enemy will fire at the position you WERE instead of where you ARE. Basically it's sidestepping. The chance is increased by:

• hellrunner trait
• movement speed (light armor and A-modded boots help)
• a lot by running mode (but it decreses aiming!)
• dodgemaster guarantees the first dodge will be successful

So basically it's a good idea to stock-up on dodge chance, and fire when Cybie is reloading, sidestep when you think he will fire.

Using this strategy, if you have Dodgemaster and enough ammo, you can actually kill Cybie with a pistol without being even scratched (there's a medal for that too ^_^)