Alzrius
The EN World kitten
This is the key.
In the last chapters of Test of the Twins, it's made clear that when Raistlin enters the Abyss to challenge Takhisis, he doesn't actually fight her, but rather he fights her (unspecified) minions. I don't have my books with me at the moment, but I recall a line to the effect of "he'd beaten her best," being used.
At that point in the books, Raistlin is fleeing back to the portal to Krynn, and Takhisis is in pursuit. He notes "Takhisis manifesting on Krynn would bring down the wrath of the heavens on her," and something to the effect that at that point "this time he would be the one directing the forces of the cosmos against her." In other words, Raistlin would take advantage of the fact that the other gods would retaliate against Takhisis entering Krynn (apparently getting himself made into a sort of field general in directing their efforts against her, and thereby paving the way for his gaining godly powers himself).
EDIT: I also seem to recall Dragonlance Adventures having language stating that the level 18 cap wasn't a total absolute, and that exceptional characters could surpass it (or was that in Tales of the Lance? I get them mixed up sometimes).
