You receive this mission from Harley Quinn on your return to the Steel Mill. She'll be tied up in the Assembly Line area. Talk to her for this mission.
Travel to the Steel Mill and make your way over to the Loading Bay how you like. In the loading bay, you want to open the shutter door heading east. Open the door and in the next room you should come to a couple of elevator shafts. Use your sequencer to hack the panel and open the doors. Stand above the left elevator and raise it (you can clear the enemies if you like) then head into the right shaft while the left elevator is raised and hop down under the left elevator. Grapple up to the ledge from there and take out the two enemies. Head into the next room.
In the next room is actually a rather messy group of enemies. Even though this isn't a stalker section, if you can I would deactivate two of the three guns in the room and then freeze the armored guy. This makes the fight much easier. Just head for the one active gun in the room and take him out. Now you're just left with a mostly normal group. Just use a takedown on the armored guy once he's free and you'll make quick work of this group.
Head to the next part of the room and you should see the box with the new gadget to your left (it will glow gold in detective mode). This ends the mission.
