When she is brownish, she is weak against Holy. When she uses Metamorphosis and changes to become bluish, she is weak against everything else, but absorbs Holy. To combat this, set your party to Ein, Serene, and Lina. Have your items as Einherjar, Leyte Claw, Infinite Arc, and Diana's Bow. The Leyte Claw can be used by Serene and Lina when she is blue. The Infinite Arc and Diana's Bow should be used as level 3 Overskills. Einherjar should only be used by Ein when Seth-Rah is brown. Finish Seth-Rah off with Disaresta when she is down to about 2,000 to 4,000 HP to get a better score.