To succeed, just zoom in on your mom
By making her appear really large on the screen, you'll trigger her to slap you.

At that moment, right before she slaps you, you'll see a red kanji character.
If you click on it, you'll perform a parry.

But in my personal experience, whenever I zoomed in on her hand and clicked like crazy, it never worked. However, when I focused the zoom on her face, the parry symbol (kanji) appeared more clearly, and I was able to get the achievement.
Hope this helps.