Got this right after the I got the 'Gel Ammo' In the room with multiple malthusians wait for 1 to pass and shot at a red explosive barrel in the middle of the room to get 2 kills and you should get it.
You can also get this while going for 7000+ combo medal and 20 combos in one playthrough. In act 2 by electric wires and gas canisters on the wall.