Davis had memory-altering techniques with him after plundering the Soul Palace, and he did manage to learn them out of pure curiosity. However, memories were something difficult to hide or change, especially when the person has less belief towards the caster who uses the technique.
It was precisely because Ancestor Tirea Snow trusted her master so much that her memories were sealed without garnering much doubt to the fallacies that filled the gap of those sealed memories.
But, the people he was facing were spirits and not humans. There was no assurance that these terrifying memory-altering techniques would work on spirits whose soul was stronger than a human's on average. Moreover, they did not have the same structure of the soul as humans did, not to mention that he had yet to seal memories of someone and replace them with fake memories.
It could be said that he possessed zero experience.
