I am Setsuna | Nidr's Legendary Spritnite: Rebellion

With Nidr in your party, go to Purikka. Go to the house on the top right corner and talk to Freyja. She will tell you that Nidr's friend is waiting at the Forgotten Shore.

Before you enter there, make sure you make a save. You also have to make sure that you're in a good condition and Nidr is at level 40 and above. Make him equip Stone Shield, this is a must! Put Avenger, Reginleif, or Lifeforce Logic for extra precautions. If you have any other boost damage/defense support spritnite, equip them to him. Buy 30 Hi-Potions or more.

Forgotten Shore is beyond Serendale. Cross the abandon village, and head there. Your 'friend' is waiting at the south shore. Talk to him and later, Freyja will come and challenge you a fight.

Never let your HP below 300, or you'll be killed. Make a wise choice between healing and attacking.

After you defeat her, you will get Rebellion spritnite and a Master's Sword.