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Be careful as his cast time is halved. He can cast Fire Storm in a second and Meteor Shower in two and a half seconds, coupled with his double damage, he can quickly kill your entire party if not spaced out.
Also he seems to summon two Charr Effigies, which are giant elemental fighters that inflict massive damage. Pull the rest of the Charr away from the Effigies and the boss to fight them separately.