While in white orchard right before starting Devil by the well quest, you need to make two manual save with one you'll go through the quest and play like always. The other save can be used to upload the game when you decide to achieve the trophy. This way you can set up the easiest difficulty and do these two quests:
Devil by the well (contract)
The Beast of White Orchard (main quest)
as soon as you slay the griffin the trophy will pop up.
To achieve the trophy you can utilize:
sword, crossbow(to slay the griffin) and food.
To achieve the trophy you cannot use:
Signs, Bombs, Decoctions, Potions, Oils and Mutagens(you need to empty your mutagen slots or you'll miss the trophy)