With Nidr in your active party, fly to Purikka and enter the house on the right to find Freyja. Speak to her to initiate the sidequest, where Nidr will ask for a sword, but Freyja says that she has someone for him to meet at Forgotten Shore. This place, if you’ve forgotten, is south of Serendale, which is southish of Purikka. Enter the area and go south to find an old companion of Nidr’s waiting for you. After some conversation between the friend, Nidr and Freyja, who shows up, you’ll be forced into a boss battle with Nidr’s master.
BOSS - Freyja
You will find yourself with another one-on-one battle, this time between Nidr and Freyja. Not only does she do some nice damage to Nidr, but she has two moves that have annoying status ailments attached to them. Thus, it might be useful to get the Shield Spritnites related to both Stone and Paralysis, although Stone is the more important of the two, since you can still attack normally with Paralysis on Nidr. It will also help to have both Provoke and Counter equipped, since the former allows you to boost defense with Momentum, and the latter can make the battle very simple.
All of Freyja’s attacks are physical, with Two-Sword Style being her basic skill that hits twice. Blunt Blow has a chance to inflict Paralysis (can’t use Techs) and Stone Slash is the one that can stone Nidr. Wind Slash is nothing special, and Double Back will counter magical attacks. Once you get her health low enough, she will begin using Rebellion, which is likely her strongest attack. The key to winning this fight is to keep your health in a good range, like over 400-500. This will give you enough time to react, in the odd case that she can get two moves in before your next turn.
If you have skills like Blunt Blow or Stone Slash equipped, using them with Momentum will heal you. Otherwise, you’re going to have to heal with items, preferably Mid or Hi Potions. The easiest way to win this battle is to use Counter and let Nidr counterattack her moves. The reason this will work wonders is that all physical attacks miss whenever Counter is used, and it doesn’t run out until you choose another action, so you can simply go do something else while the battle will play itself out.
After the battle is over, you will get both the Master’s Sword for Nidr, as well as the Rebellion Spritnite, and this sidequest will be finished.