This is awarded for killing only the two mandatory Box heads in chapter 7.
The mandatory box heads are the first 2 you encounter To avoid killing the rest have a freeze bolt and shock bolt ready.
Use the freeze bolt once the box head catches up to you after retrieving the replacement valve.
use the shock bolt in the doorway before the valve you have to replace to turn the gas off.
you should have enough time to turn the gas off and run away without killing anymore box heads.