*POSSIBLE SPOILERS FOR STAGE 2 OF THE ECHO SIX DLC PACK*
In order to get this achievement you need to hit someone with the crucible. This is the molten metal dispenser you used thrice to defeat the Nemesis in the second stage of the Echo Six DLC. This is only possible on the Foundry stage.
First, a couple things. Every time I started this stage in the Team Attack mode, it froze. This map doesn't appear in the other game types, generally. What you have to do is press Y when selecting a stage to change your map preference to Echo Six Pack 1. For some reason this makes the stage magically appear in the other game types. I used Biohazard but the other types work (except Team Attack) and had no problems with freezing. Make it private if you want no interference.
Once the stage starts you'll need to both go to the factory area. There are three circular areas that have a grated bottom and a tube above them. Any of these will work. You'll need to get to the upper level using stairs or the ladder and look for the switch for one of the tubes. Have your partner stand under the one you find the switch for and then press it. They'll die and you'll get the achievement.