Well, even if he wanted to deal with Niflheim immediately, he didn't have the means to do so. After all, he didn't even know where they were located.
Xiao Zhen had told him that they would be in the vicinity of LA, but that wasn't much to go on. He wondered if it was even possible to rule out the ocean, but he quickly scraped that thought.
If the rest of earth was simply a playground for Damien, the only place he had to be wary of was the ocean. Even before the World Awakening, more than 80% of the ocean was unmapped and unexplored. The world's terrain had changed since then, and nobody knew what had happened to the ocean.
The only known fact was that it was dangerous. The cultivation of beasts was much simpler than that of humans in the sense that they didn't need to level up until they reached 4th class. As long as they ate other beasts, they'd gain power.