In the end, Dilan decided to learn the Old Librarian's Essence Cultivation technique.
It was the Old Librarian's Legacy and his most prized possession when he had still been alive. Before he separated a Fragment of his Soul to deploy it inside the City of Trials, the Old Librarian had altered the technique to allow any Ascender to reach the fourth Tier as long as they practiced the Essence Cultivation technique.
Every Ascender's talent was different, meaning that an Ascender with great talent might advance to the fourth Tier within a single year, while an Ascender with the worst talent wouldn't even be able to reach the first Tier in the same duration.
But despite all of that, the Essence Cultivation technique allowed one to follow a different path to advance in the Tiers rather than the endless killing that had been a pre-requirement to become truly powerful.