Story-related and unmissable. This trophy is awarded for defeating Father Gascoigne, who will be the second boss that most players encounter. He is the final boss of Yharnam, and is found after finishing the sewer area. He uses swiping attacks as well as gun-based attacks, but fortunately you can use the gravestones in the area to block shots from his gun. After he loses about half of his health, he will transform into a beast and become much more dangerous – he attacks quickly and heavily in this form. Try to riposte with your gun, and stick only to swift attacks with the one-handed version of your weapon throughout this fight.
See the following video guide: