When walking up to the field where you battle the Shrieker go left as soon as the battle starts. walk up to the river (Not the small puddle) and stand in the water. Wait untill the shrieker air attacks you and hit him with the axii sign (if levelled up high enough (I had level 3)) it will stun the shrieker causing him to drop in the water and drown instantly. I did this by accident after dying three times in a row on Death march difficulty.