DOOM + DOOM II

Doom 1 > Level Select > E3M3 on I'm Too Young To Die difficulty > Short video (not mine) shows the very short route. Definitely pick up the invulnerability to avoid gibbing yourself.

Grab BFG, turn around and a single enemy will be right in front of you, or at least very nearby.

If you are doing this on Xbox, you will need to plug in a keyboard to be able to type in cheats

Load up Doom 1, select E2M1 "The Shores of Hell" on the level "Deimos Anomaly" and select the easiest difficulty "Im Too Young To Die" as this will spawn a single Former Human enemy on the first right you take

Type in "IDKFA" to get max ammo for all weapons, select the BFG (oh yeah) take your first right and fire anywhere near the enemy to kill it and the achievement will pop soon after

(You can do this on loads of levels, this is the way i did it)