Everyone is always arguing over the "best" abdominal exercises. Some stick with situps and crunches, others go with planks, and still others get fancy specialized equipment involved. Me? I say learn to squat, deadlift, lunge, etc. Nothing engages your abdominal muscles better than a heavy deadlift, squat, or lunge. Why? Because our abdominal muscles are not meant to primarily bend, flex, and twist our torso. Like Dr. Stuart McGill has proven, our abdominal muscles are primarily responsible for transfering energy from the ground, up...through utilization of "super stiffness."
The overhead lunge is a great variation of a lunge that significantly activates the core musculature, as well as improves shoulder girdle strength and stability. As you will see once you try it, the lunge portion is not usually that difficult. Rather, the difficulty lies in maintaining a stable, erect spinal column and preventing your arms from moving at the shoulder. Try using no weight at first, and then progress to very light weight next. Don't even think about trying a weight anywhere close to what you can press, unless your are He-Man of course!