To find the specters, you first need to complete Gwenora's side quest. You will meet Gwenora early into the game and then later in the Cemetery. From here you can start her side quest. You need to find locations based off of the pictures she provides you with. After finding all the locations, she will give you the location of the Specters. For the fight, you can activate Instant Victory from the options and then select it when it is your turn to defeat the boss.
Timestamps:
0:00 Cemetery (Location #1)
0:45 Ikenfell Grounds (Location #2)
1:08 Library (Location #3)
1:46 Southern Dorms (Location #4)
2:22 Northern Dorms (Location #5)
2:47 Twilight Tower (Location #6)
3:07 The Roost (Location #7)
3:33 Gardens (Location #8)
3:59 Spelltower (Location #9)
4:28 Dueling Hall/Allcat Lair (Location #10)
5:38 Spirit Oval (Specters Location & Boss Fight)