DOOM 3: BFG Edition

If you really want to get the achievement as fast as possible:

Right after you get the Soul Cube (in Hell) the next level starts off with a narrow hallway where about two dozen spiders emerge. Every time you kill 5 of them, you can use the Cube once. Save before you start killing them, kill all the spiders using the Cube whenever possible, then reload & redo the section. (Note that the Cube usually only kills one enemy per use, but it can be more if it flies through more enemies by chance.)

If you do this 5-6 times you should have killed enough spiders with the Cube, to make it pop.

If you have the soulcube and kill 5 enemies with normal weapons, the soulcube will be charged up and ready for use. It is an instant kill and fills your health. After the usage you have to redo the procedure. You get the soulcube after the fight against the guardian in hell.

You can get this either normally on any difficulty after the fight in 'Guardian of Hell'.

You can also get the Soul Cube after you complete the MAIN CAMPAIGN on 'nightmare' difficulty so you can get the 'Soul cube' right at the start of the game on the 'Nightmare mode' so you can get more than enough kills through the campaign by regular usage on the nightmare difficulty, it also means you can try to get your nightmare achievement too

Just remember it takes 5 kills to be able to use the soul cube, I highly recommend trying to save the soul cube for big enemy's like the Hell's Knight's or general boss's, I highly recommend saving it for things like the Arch-vile since it will frequently bring back demons that you have previously killed. Also I believe all kills with the soul cube might be carried over but I can't confirm this.

Also if anyone has any negative feedback please post a comment, I'll try to amend.