Well, every time you destroy each of his lens in phase 2, it explodes and deal to the party 40% of each individual max HP. And when you pass to phase 3, the boss and any remaining lens explode - boss for 25% HP, and each remaining Lens adds + 40% . You can survive one single Lens remaining with the Boss.

Do you have another source of haste, Heavy Regen or +Resist for this fight? You could use a Medica with Hastega or Heavy Regen as a compromise, in place of DG. And now Arc has a +Res Medica. Priority should be hastega > resistance > heavy Regen (because you should have healers to spare).