The basic idea is to soften the CyberDemon up with rockets, the BFG, or plasma rifle, save, and then slowly wear him down until one punch will kill it. The first place that you’ll encounter a CyberDemon is in the last level of Doom 1, Episode 2, but it will also appear periodically in later Doom 1 episodes and Doom II.
This is a pretty annoying trophy, but there is an exploit you can use to make it easier abuse the save function.
