The Venom-X becomes available after clearing the middle hive in Area 3. Once you clear the hive, the Venom-X can be picked up inside of the room. All members in your group are able to pick it up for free. The gun is extremely powerful, but comes with very little ammo, and additional ammo for this weapon is picked up from dead enemies who drop a small glowing egg. The following video will show the weapon location in detail:
Once you have the weapon, you must get 50 kills with it, and the achievement will unlock. The achievement is NOT cumulative, and will keep count. This means that players will probably need to take turns takign the Venom-X ammo until they unlock the achievement, as you need a good amount of ammo to get 50 kills. Flares will help with this achievement. Also, the Final Boss has regenerating health, so theoretically you can boost this in the final area of the Nightfall map. Just continue getting kills until you have the achievement, then finish off the final boss although this method may take 40+ minutes.