Exactly what do you need to do to get this hidden achievement? I've participated in three rook attacks so far. I've quaffed Rook Balm 5 times and used Essence of Rookswort 1 time and still no badge. Do you have to use it a certain number of times?
Are you participating in dice roll induced attacks or the real attacks? Essence of Rookswort is the correct item and using it some time before the attack is over then contributing to defeating the rook should work.