After an extremely brief level of Delta Labs 4, you will travel to Hell itself. At the end of the Hell level, you will enter an arena with the Guardian of Hell. The Guardian is blind and sends out Seekers, which are flying creatures that light the area to locate you and call the Guardian to your location.
While dodging fireballs that roll across the ground toward you, shoot the Seekers (with a shotgun or machine gun) which appear 3 at a time hovering above you. With 3 Seekers dead, quickly shoot the blue-light above the Guardian's back with a chaingun or plasma gun. 3 more Seekers will quickly reappear, so you can restart the process 2-3 more times. On the 3rd or 4th attack on the blue-light above the Guardian's back, the Guardian of Hell will die. Then you will receive the Soul Cube, not to mention an unlocked achievement for 15G.