-( Story related, cannot be missed )-
After you have returned from Whitechapel and attended your sister's funeral, you will return to Pembroke with dire news awaiting you. Harriet Jones, who you spoke briefly to, has been slain and her supposed murderer, Sean Hampton (who you saved earlier) has fled back to his flock in the Docks. Managing through the dredges of the sewer thereafter, you will come across Fergal, briefly introduced to you during your time in Whitechapel, only this time he is your enemy.
Be aware that the dodging mechanic is CRUCIAL to surviving him. He is a Vulkod, a brutish vampire that lives off of instincts, and so his attacks do hit harder than most (especially if you are of a lower level than him because of the "Not Even Once" trophy you might be trying for). Dodge his one-two or one-two-three swing attacks and then go in for only two hits if you can. By this time, you should have the Barbed Cudgel two-handed weapon, so your stunning potency might be decent enough to bite him a few times.
His second and third phase attacks don't differ greatly from when you start. He will have a roar attack that disperses shadow damage around him. He may also employ shadowy dopplegangers that, though you can't damage them, can certainly damage you. This is why I suggested only two hits at a time, as you still need to conserve some of your stamina to get away from either of these attacks. With enough effort and certainly enough attempts, Fergal will fall and this trophy shall be yours.