Okay guys heres the challenge...
Max lv in Krynn was 20th and that had only been obtained by Raistlin, who defeated him. For specialization, Im torn. He didnt use a ton of undead minions, however he used a very unique form of magic jar to preserve his life over the centuries. Now he was the Master of Past and Present with the Tower per the spectres that resided there so Im assuming he created them. However, I dont see him as a necromancer per se.
Thoughts?
Without using Dragonlance 3.5 materials, I would take a look at the Tome or Magic or the 3PP Secrets of Pact Magic & Villians of Pact Magic by Dario Nardi. The is even conversion notes to Pathfinder for the 3PP stuff. I would make Fistandantilus an aspect/spirit/vestige, hence the master of past and present - then allow a mortal spellcaster (most likely a soul weaver) named Fistandantilus to bind the vestige Fistandantilus to him for the day. This allows Fistandantilus as a mortal spellcaster to be only 18th level but have a lot more omph! compared to an average spellcaster. It also allows Raistlin to bind the vestige Fistandantilus at the same time. Just a rough idea.
While AD&D statted him as 23rd level Black Robe I would make him as 20th level Wizard with either Necromancy (soul-stealing, etc) or Conjuration (demonology, planar travels) as chosen arcane school.
Remember that Necromancy is more than mere animation and control of Undeads - it manipulates souls and lifeforce.