Try to get all this done in under 50 seconds work with 10 seconds rest. No equipment is required, just a couple magazine covers to create some friction.
Repeat 1 time through, so you do everything twice. Once you get used to it, increase the amount of repetitions.
While you are resting, remain in the plank position. This is what makes the Killer Abs workout tough. Keeping your abs flexed and intact is tough, but will strengthen your core more. Core exercises where you constrict blood flow to the abs area will help increase muscle definition in that region.