Immediately after earning the "Celestial Emissary" trophy, look around the room for a window that you can break. Break it and go through it to enter a balcony area. Continue along the path and go down an elevator to enter a new area called the Altar of Despir. There is a big, motionless creature at the far end of the room. This creature is Ebrietas, Daughter of the Cosmos. Attack her to start the boss battle.
This boss is extremely large, and like most large bosses, your best strategy is to stay directly behind her, in close range. When directly behind her, you can repeatedly attack her without her having much of a chance to hit you at all. As long as you avoid her (very deadly!) frontal attacks, you should be fine. She moves quite slowly, so staying behind her should not pose much of a challenge.
Once she gets low on health, she lets off an aura that slowly depletes your health. You can mostly ignore this, as the health drain effect is slow enough that it shouldn't cause you too much trouble. Just keep your health up. You will earn the "Chalice of Isz" trophy at the same time as this trophy. See the following video guide: