Story-related and unmissable. This trophy is awarded for defeating the Witch of Hemwick, who will be the boss that most players encounter after Vicar Amelia (see the "Vicar Amelia" trophy). This boss is sort of a "puzzle boss" rather than a straight-up boss. There will be a soot monster that appears as soon as the boss battle starts, but just ignore it – killing soot monsters here will just cause more to spawn, and they move slowly enough that they should cause you no real trouble. Instead, focus on trying to find the witch – she is a small, hunched old lady, and she is very hard to see and lock onto.
Once you drain her health enough, another witch will show up, and this new witch will use some (easily dodged) magic. Just repeat this process until all witches are dead. See the following video guide: