Surviving this baptism requires an understanding of the essence of the Law of Fire, harnessing its energy to fortify rather than destroy oneself. Usually, a grasp of the Laws and a robust foundation would suffice to survive such a trial.
If an Apotheosis Master had reached an Extreme Law Fragmentation mastery over the Law of Fire, surviving the Fire Tribulation would not have been a problem. Therefore, with his Medium Law Integration mastery, Cain should have faced no challenge at all.
However, the flames assailing Cain were unlike the ones that challenged all other Apotheosis Masters in Aether. In them was imbued with a far more profound and potent Law of Fire, showcasing the might of Extreme Law Integration or perhaps even ascending to a realm previously unexplored in Aether.