Question: what are dagannoth weak against?
Answer: Dagannoth Rex - Attacks with Melee and is weak to Magic. Unique loots include: Berserker ring and Warrior ring.
Dagannoth Prime - Attacks with Magic and is weak to Ranged. Unique loots include: Seers ring and Mud battlestaff.
Dagannoth Supreme - Attacks with Ranged and is weak to Melee.