The first place you can get this is in Erebus-Level 3. Right after you get the Super Shotgun, you will need to grab a power cell and take it back downstairs. On your way there, you will enter a room that will change to a red glow and many zombies will appear. Get them all close together and then activate the artifact. Start killing them with your weapon of choice. I'd recommend the regular shotgun as it doesn't have to be reloaded after every shot but will still 1 shot them point blank. If done correctly, you will earn your trophy.