On the mansion level, after you go through the infinate spawns at the front doors - watch The Darkness do its thing as it takes down a helicopter. You will gain a brief period of control during the ensuing madness right after you go through a checkpoint. You will be facing a wall in a corridor of the mansion and the corridor will turn right. In the room that joins on to the corridor will be 7+ enemies. As Yuriam and Hurball have suggested, speed is not of the essence here; take time to shoot out the light and ready yourself, then step round the corner and simply unleash your black hole in the center of the room.
I got this achievement and the Gunslinger achievement in one.
Mojo I Uk adds that if you don't net the Gunslinger achievement at the same time and if you're quick enough, you can head in to the room and finish off the survivors with your weapon of choice for another shot at this possibly linked achievement .
On a final note Angrymagnox suggests;
'After 20+ tries the achievement hadn’t popped. As a last resort I cleared my system cache and completed it offline. It popped first time.'