During
Act 2, after picking up the
Chronobomb and using it to slow time in order to drop through the fan, you'll be in a circular incinerator room with a lava hole in the center.
You will then be ambushed by about 12 robots. Press
to push these robots into the smelter in the center, using the Chronobomb to slow time if you get swarmed.
You can also run around in circles until they fall in the lava themselves.
After 7 fall into the smelter the achievement will unlock.