Story related and cannot be missed.
The Guardian is the boss you must defeat within the Hell levels to gain the Soul Cube weapon.
The absolute easiest way to kill him would be to enter the portal with the BFG equipped and let the cut-scene finish, then let off a fully charged BFG shot right at his blue energy sphere; DEAD. If he doesn’t die, continue the fight like described below.
However, if for whatever reason this doesn’t work for you, then this is the second best way: try to utilize the pillars and corners as cover and shoot down the “seekers” first. They look like flying squid things with flashlights. Once you kill these, aim for the huge energy type sphere above the boss itself and you should have no problem defeating it.
Here is a video of the battle by AlanWilderDM: