The best way to get this achievement is to use your Ogryn with his Heavy Gun. Also, use his "Point Blank Barrage" skill.
Choose a Mortis Trial from Sefoni. Ideally, you should be on the "Carnival" map, the urban map. Before starting, just near the starting point, you'll see a small area where you need to wait for a horde. As soon as it starts, wait for the horde, and when it arrives, throw a Frag Bomb and then fire with your Heavy Gun. If you run out of ammo, use "Point Blank Barrage."
Another way to get this achievement is to complete the "Archivum Sycorax" map. At the end of this map, just before the extraction point, a horde will try to prevent you from reaching it. This is your moment to use your weapons.
(Remember to play on a higher difficulty level to have more enemies in the horde and increase your chances of success.)