DOOM 3

This is the boss found on Central Processing: Primary Server Bank. The best option is to have a fully charged Soul Cube going in. Fire off the Soul Cube if you have it then blast the boss with your plasma rifle and rockets. On difficulties easier than Marine you should be able to kill him extremely fast.

This is mostly a guide for your NIGHTMARE run but can apply on easier difficulties as well.

Sergeant Kelly, now turned into Sabaoth, can be found on Central Processing: Primary Server Bank. Save after exiting the rotating bridge, just before the door at the end of the hall. Prepare a quick switch to your Rocket launcher and equip the soul cube. (You absolutely want a charged cube for this as the tendrils from his BFG shots can kill you on nightmare long before you are able to shoot them to disrupt them.) Run into the room moving to the left pillar, and immediately, as the door containing Sabaoth starts opening, throw the cube and switch to your rockets. Pop behind cover every shot to prevent him from firing the BFG, and he will go down in 5 relatively quick rocket blasts. Below is a video.

*the pillars can electrocute you if not quick enough but I've done this on 3 different versions of the game on all my nightmare runs and it works quite well.