In short: The way to get a strong body is to practice high-quality movement. Often.
DISCLAIMER: Doing a bunch of reps of these will get you REALLY GOOD at doing a bunch of reps of these! The movements I'm demonstrating in the videos are generally intended to be used within a longer workout session combined with other movements that hopefully involve some weight training or bodyweight movement and mobility work. BUT if you are hell-bent on a 30-day "abs" challenge, the videos I made are a few suggestions for things to do INSTEAD OF sit-ups & crunches.
For you, 30-day Abs Challenge friend(s)!
(Feel free to email me if you would like suggestions on how to modify this movement if it feels too advanced.)