While attacking, it channels to random locations that spawn Sodden Growths. All Growths must be destroyed in order to damage the Sodden Effigy itself (that will be immune otherwise). Also, normal Drowned spawn during the entire fight. Targeting the Sodden Growths is your top priority before single targeting the Sodden Effigy with a clear damage window. Use AoE (Area of Effect) skills on normal Drowned if they get out of hand.