Start Phase one by praying melee. Keep this prayer up throughout all of Phase one. If you are not within melee distance during her "Pause" you will be hit with a range attack. You can choose to switch your prayer to protect range or simply enter melee distance. Anticipate when the Fire Line will show up to avoid taking unnecessary damage.
During Phase two, pray melee until you see her lance hit your character. Immediately pray range to reduce the damage you would have taken. After two melee auto-attacks, Vindicta will fly to a random corner of the arena and use a Fire Line toward the targeted player. Once again simply move before out of the way to avoid this attack. Repeat this process, keeping a close eye on when her range attack occurs and when she does Fire Line until she is defeated.