Hunting
Rui did not want to run into a swarm of Senior-level hornets. The fact that they had almost caught up to him prior was quite a close shave. Unlike in the Shionel Dungeon where he had come up with specific targeted solutions towards the monsters, he didn't have that here.
His goal wasn't to massacre monsters in the first place, it was to avoid them while he searched for the Hypnomaster. Wiping out monsters and animals the way he had in the Shionel Dungeon would merely drain his energy for no real purpose, even if he did have the means.
He was sure that his Phantomind Void technique would work on the hornets but it was not a viable long-term solution. Furthermore, he figured it was best that he didn't rely on any particularly mind-consuming techniques. In an environment where mental stamina was compromised, mental energy was something that he ought to avoid spending.