Polter Armor (Soul Eater; Dart's Ultimate Weapon) in Fort Magrad
Polter Armor is a side quest that takes place in the Snowfield area which you went through while on Disc 3. Depending on how far progressed you are in the game will determine how you want to go about reaching this location. If you've gotten Coolon and can fly around the world map, go to Deningrad and then head west to Kashua Glacier. Travel through Kashua Glacier to the Snowfields.
Once you've made it to the Snowfields area you'll want to slide down the slopes next to the sign which warns you that things are slippery. You'll find yourself at the bottom next to a green teleporter similar to what we saw in the Magical City of Aglis. From this location go up a few screens until you find a save point next to a stage like area.
All you need to do is approach the statue on the stage and a scene will trigger with Rose prior to the fight starting. Depending on your character's level and current equipped armor/accessories determines how hard this fight will be for you. If you're above level 30 using your onslaught of Dragoon attacks will decimate this boss.
The only thing that you actually need to worry about during this encounter is your timing on defeating the Polter Sword. Upon defeating it, it'll use Can't Combat and knock out a member of your party - if you want them to get EXP you'll need to be out of Dragoon form on at least 1 person so that you can use an item. For that reason I don't recommend you use Special on this boss.
Your reward for beating this boss is 6000 EXP and Dart's ultimate weapon, Soul Eater. This weapon might have the best stats but it comes with a major flaw, it saps health from Dart each round similar to Poison. You can negate this side effect by equipping Therapy Ring on Dart (acquired from Hellena Prison earlier in the game) but that means he will lose his valuable Accessory slot. A big no no for me, I prefer Ultimate Wargod in its place.
In addition to the Soul Eater as a reward, search the chest in the area you defeated the Polter Armor to find Armor of Yore, a decent chest piece that allows you to avoid many different negative status effects!