I don't really see a way to deal mental stress to a human without going lawbreaker.
From the 4th law in YS: "Here, enthralling is any effort made to change the natural inclinations, choices, and behaviors of another person".
It's so broad that things like confusion/befuddlement spells might fall under it, giving someone mental fatigue might fall under it (you're changing their behavior in the sense that if they were not fatigued, they would act differently). A stretch, maybe, but it goes to show that at best, you're in grey areas, and you'd need a clear conversation with your GM about what is and is not lawbreaker for your game so that you don't start with one expectation and get slapped with a lawbreaker that you weren't expecting.
Now, mental stress in other efforts, such as one of my player's recent enthrallment of a fey beastie, is just handled like normal spellcasting. He used spirit, the creature resisted with discipline, it lost, he told it to go eat the vampires.