Shhhh
When it comes to making a human go insane, the easiest method involves just whispering into their ears.
At random intervals, you just need to whisper something, such as the human's name, into their ear.
They'll become extremely paranoid and doubt their own senses. You don't even need to do it for a few hours as they'll go completely insane long before that.
A few other suggestions:
-"I heard your prayers, but I'm busy."
-"Behind you."
-"You're going to die in 7 days."
-"What year is it currently?"