Fistandantilus, like Raistlin, or even Cthulhu for that matter, are the sort of characters you can probably stat out any way you like as long as they end up at or just beyond the top end of your games power scale. (In Cthulhu's case, go way beyond the bounds of sanity with your stats and your set).
Fiction is rarely written in such a way that sets out exactly what a powerful character can or cannot do. If they need to be totally badass, they are written about as such. I doubt Weis and Hickman sat down and said "You know, this guy is pretty powerful. But we know that Raistlin eventually whoops his arse. I also figure that the Kingpriest is about out of his league, so lets peg him at about level 22 instead of 27".
The only reason to stat out what such a character can do is if you actually want your players to have a shot, however unlilkely, at killing him at some point. It has been pointed out elsewhere that the moment you give someone hitpoints, the players can find a way to kill him/her/it.
The actual numbers used in any game depend entirely on the way the DM runs the game. If your at least trying to stay consistent with the Dragonlance campaign world, then Fistandantilus is supposed to be one of the most powerful wizards that ever lived. So set him up to be the most powerful villian NPC your campaign has yet seen.
Getting back to the question at hand, just be aware tht depending on the timeline, Fistandantilus either gets killed by Raistlin, or he goes into the Abyss and manages to take on Takhisis. So he should certaintly have enough power at his disposal, either through his spells or through his collection of magic items / artifacts, to handle anything short of that. For low power campaigns, level 22 should be plenty. For higher power games, you may need to kick him up to the 40's, or say a few levels above the typical avatars of the gods. He should also have a 'retardedly high' set of stats. His Int should be boosted due to advanced age bonuses. Depending on how recently he used the Bloodstone, he may either be suffering from stat penalties to Str/Dex/Con due to age, or he may be in his prime. He may also have other powerful items on his person at your discretion.
END COMMUNICATION
