Another quick video; this one shows how to kill an imp with only one shotgun shot in Doom 3. Stay some feet far from the imp, far enough so he can throw the fireball and you can strafe. Keep your run key (shift is default) pressed and, soon after he throws the fireball, strafe left or right, run towards him and shoot him on the chest. It should help on saving ammo, something important on harder skill levels (like Veteran).
