The first Cyberdemon encounter is on the level E2M8 (last level of the Shores of Hell missions), make a save state when you get here. Use the plasma rifle as it is the easiest to hit the demon with it. If you have full ammo (600 rounds) you can shoot the demon until you have roughly around 360 rounds left and then switch to the pistol to finish it off. It will take a lot of handgun bullets even at this point since the handgun is a very weak weapon. Below is a no commentary video of how this works.