Level 20 is (was?) the final level before becoming a Demigod, or epic. That's why you can, "arbitrarily define his level as 20."
These days, at level 21 you may take the Epic Destiny Feat (RPG.SE - How does one become a god?).
New Feat: Epic Destiny
You have a destiny beyond that of other adventurers.
Prerequisite: 21st level, any other requirements listed in the epic destiny's description.
Benefit: Choose an epic destiny. You gain that epic destiny's benefits at 21st, 24th, 27th, and 30th level. When you have this feat, you do not gain additional feats at 24th, 27th, or 30th level. –wizards.com
According to user Ignominia at giantitp.com, Raistlindantilus was level 27 in the D&D module Legends of the Twins, by Tracy Hickman. I do not own that book so I can't verify this:
Raistlin @lvl 1 - Towers of High Sorcery pg 4
Raistlin @ lvl 6 - War of the Lance pg 245
Raistlin @lvl 27 - Legend of the Twins pg 84
The real question, for which I have no answer:
I doubt Terry Phillips played this character even up to level 20.
"I've played d&d on and off for 20 years, and I've only gone from 1-20 once." –(user) alittletooquiet; Has anyone ever made it from level 1 to 20? –reddit
Also, he didn't "die" he was defeated. I do believe that whether or not he 'ascended' is left up to the reader. All deities are immortal*:
Immortality
All deities (even those of rank 0) are naturally immortal and cannot die from natural causes. Deities do not age, and they do not need to eat, sleep, or breathe. The only way for a deity to die is through special circumstances, usually by being slain in magical or physical combat. Deities of rank 1 or higher are not subject to death from massive damage. –d20srd.org
I cannot recall if Takhisis heals him herself every day or not. Regardless, she may not have had to:
Divine Recovery (24th level): The first time you are reduced to 0 hit points or fewer each day, you regain hit points equal to half your maximum hit points. –dungeonanddragons.wikia.com
*all of this begs the question of if a Demigod is a deity, which WAS a yes.
In every edition of D&D except 4th, demigods have been the lowest rank of power of the divine god scale. They are true gods, just the weakest of the true gods. –forgottenrealms.wikia.com
Stats:
STR: 10
INT: 17
DEX: 16
WIS: 14
CON: 10
CHAR: 10 [15]
–angelfire.com (not sourced)
Here are his stats from the hardcover book:
Str 10; Dex 16; Con 10; Int 17; Wis 14; Cha 15
They're the same in the original module where he was introduced.
–sorcerers.net (unconfirmed; I do not own that book either)
The only discrepancy between these two is his Charisma stat: 10 or 15. It is unclear to me if he was Raistlindantilus, "in the original module where he was introduced."
It is widely believed that his stats changed considerably after taking The Test (the only one that's apparent is his shattered Constitution), and then again after he used the Bloodstone on Fistandantilus.
Furthermore, if he truly was a Demigod, at level 21 he would have been granted the Divine Spark class feature, giving him the choice of increasing two ability scores by 2 each.