What is it exactly that makes a High Intensity Interval Training (HIIT) workout a challenge? It's the gradual build up that stops the muscles from recovering fully, that builds up body temperature and makes each set feel a little bit harder. It's the pressure that keeps your heart pumping and your blood flowing through your body. We're talking Cardio Supreme here. Do it! No hesitation and aim for the highest rep count possible in each exercise of each set (apart from plank, of course) and, oh, yeah, EC it!
Extra Credit: 1 minute rest between sets.