Doom 3

I got this in hell after the first room into the 2nd room with the lava and 4 jump rock things with armor in middle, previous room spawn 2 cacodemons and run past the first 2 imps in corridoor, get into next room and kill the 3 flying heads and then the cacodemon, walk past the tile half way and you will spawn 2 more imps, so now you have 4 imps there, damage both from far with double shotty then shoot both with 1 shot close range.

RoE campaign Veteran Difficulty, hell level at beginning

Relatively easy to do near the beginning of Doom 3 campaign. It's also the very first time when you will encounter two imps attacking you at once, a perfect opportunity for this.
In the 3rd level of the game, Administration UAC Division, two imps will tear through a door. Soften them up with the machine gun, backtrack a bit inside a narrow corridor, switch to your shotgun and blast away! Took me about 5 to 6 tries because of the extreme pellet spread from the shotgun, but it is perfectly doable in this level. This is the same level where you first encounter the Pinky demon and also the first one where you can find the chaingun.