I want to try for the achievements. I got the x1 achievement, obviously, but how do I get the others? Do I have to take down a certain # of enemies without taking damage? Is there a hidden button somewhere? How does that work?

I'm just trying to practice my 2-digit and 3-digit multiplication, but enemies spawn so fast I'm mobbed before I even have a moment to think. Even if I manage to bang out a few correct answers, each one only kills 1/3 of an enemy! It shouldn't be getting this fast until you've been playing for a long time....