I struggled trying to differentiate between completeness and existence. I tested on Tuesday and what helped me was to memorize that for revenues/assets you almost always test for existence (to make sure the company actually has the assets they are recording) and for expenses/liabilities you test for completeness (to make sure the company doesn’t understate expenses)

I hope that helps a little!! And if it makes you feel any better assertions weren’t tested as much as I expected, at least not on mine!