Killing the Cyberdemon in Doom 3
The final boss of Doom 3 is awesome. The Cyberdemon is huge, cool looking, and intimidating. You have to use the Soul Cube to harm him, charging it with the demons spawning out of the literal Hell Hole in the middle of the arena.
Luckily, the BFG 9000 makes very short work of these demons. Even a non-direct hit on the ground will likely kill a demon and further charge the Soul Cube.
Just goes to show that anyone wielding both the BFG 9000 and Soul Cube will kick demon ass for all eternity.