Abs

About the training, i dont know much about specific ab training, but i dont think they work much diferently off your other muscles. Mikasa in shingeki no kyojin spends most of her time hanging in wires, so a good way to emulate that is with rope or silk aerial training (or even a pull up bar, if you dont have those equipments) and do leg raises, catchers hang abdominal curls, l-sits, flags, winshields will hit pretty much every single muscle on your abs, (and work your grip strength as well). Consider maybe tring some back lever variations to balanced core strength

The best idea is to cicle the intensity and volume of the workouts, doing,lets say, 6 reps x 3 of each exercise on the first 3-4 weeks, and after this, doing 12-15 reps x 3 sets, 3-4 times a week.

Well, this is just an opinion of how i would aproach this.