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.

You must kill Seth-Rah before she transforms (reversing resistances). The first form of Seth-Rah is only weak to Holy and neutral to Physical, and has very high resistance to other elements. Your main weapon here is Rosier. Do not waste it before the final battle. For Ein, though your powerful diviner is Holy, Rosier will still deal more damage since you are only fighting one enemy. Rosier deals three hits. Use ExL only when Seth-Rah falls below 4500 HP. Disaresta will be this powerful against the primary Seth-Rah. Fanelia is not really necessary unless you are in a hurry. Fia is almost a must in this battle. Use her Level 3 Overskill using Rosier for great damage. Also, stack up some books for a full-party heal. Seth-Rah's skills hurt the whole party For Lina, Spiral Arch skills are Holy and Lina's Overskills are very strong against single opponents. Keep Diana's Bow (Lightning property) in your inventory in case Seth-Rah changes form. Bows can be used effectively by anyone except Serene. Serene is very useful in this battle. Have her use Rosier. She deals three hits becayse she has the highest strength in the party. Keep an Infinite Arc in case Seth-Rah changes form. Cierra is a witch, but have her use a rapier instead of a wand. Cierra using Rosier will deal three hits dealing magic damage. Cierra has powerful magic. Keep a Southern Cross to decimate Seth-Rah if she makes the mistake of transformation. The battle will not take long and it easy to get an "S" rank. When you get an "S" rank at this battle, you will get the powerful Longinus early on your next adventure.