Wallace remembers the five skills that he can learn in the future. The Void Phase, Matter Disassembly, Essence Reconstruction, Astral Core Formulation, and Chasm Trample.
With all these skills, he may not be the strongest magician but he will be the most versatile one and he would be able to contend against the Celestial Beings at the northern continent.
That's right, after integrating himself with a Celestial Fragment to form the Astral Core, he understood a lot of things about their existence. It was also due to the Empire's Prince's logbook that explained almost half of it.
The Celestial Being was the perfected creature that the Prince wanted to use against the empire... But they became too powerful and they no longer listened to his instructions pushing him to flee from the northern continent.