Here’s what to do:
Make sure you’ve selected Torbjörn and you’re playing inside Quick or Competitive modes.
We unlocked this in King’s Row in Hybrid mode, defending against a Payload escort. You can really do this on any map that has an escort component to it, as players tend to bunch up at choke points.
Torbjörn’s super, Molten Core, gives him increased health and attack, and if you’ve got a turret deployed, that’ll automatically be upgraded to level 3. Play as normal to build your super, and place a turret in an area where it can easily shoot at enemies. Try to place it where enemies can’t easily destroy it, either — a bit back from the action is best.
Use your super when you’ve got at least four enemies nearby.
The Achievement will unlock when you’ve successfully killed — or assisted in killing — four people during the duration of your super’s run. Said kills can come from your turret, your rivet gun or your hammer (or any combination of the three).
Voila! Here’s how it all looks, keeping in mind we made the video using XboxDVR.com, so the Achievement doesn’t actually pop on-screen.