Ensure your RDM is as high as possible. The best thing to do in this fight is eliminate Arngrim first. Cast Sap Power on both foes as soon as possible. Poison Arngrim, and get rid of him however you can. Use Fairy Tinctures to recover the high amount of damage inflicted.
While attacking Arngrim, avoid Hrist. Both are extremely fast, and the battle field is narrow, so plan moves carefully. Once Arngrim is gone, begin to attack Hrist. Cast Sap Power whenever her damage becomes normal again, and attack from as far away as possible. After losing 50% of her HP, Hrist will use her Soul Crush Nibelung Valesti, which is pretty much certain death for one character. Revive them if possible, and continue to chip away at her HP. Attacking from behind is a good idea, as she cannot block attacks. Attacks that launch will also prevent her blocking you, as does Fire Storm. Stay sharp, and attack whenever possible, and you should prevail.