The Farron Greatsword deals 20% bonus damage against the following enemies: Pus of Man, Darkwraith, Monstrosity of Sin, Harald Legion Knight, Enemies with Red Eyes. The Farron Greatsword deals 20% bonus damage against the following bosses: High Lord Wolnir, Aldrich, Devourer of Gods, Dragonslayer Armour, Iron Dragonslayer (Mini Boss), Darkeater Midir. I think these are all the "Abyssal Enemies". Feel free to correct me if I forgot any. I still have my doubts with Iudex Gundyr in his second phase. Does he becomes weak to Farron Greatsword when he transforms?