Core exercises and cardio intervals are by far THE best boot camp exercises for flat abs. The trick to getting results from this awesome combination is to use the tabata protocol and to tack this protocol at the end of your total body workouts or at the end of your regular cardio workouts.
The tabata protocol also known as tabatas is 8 rounds of 20 seconds of work followed by 10 seconds of rest. You simply work for 20 seconds then rest for 10 seconds and repeat 8 times. It takes 4 minutes and is very intense but also very effective for fatloss.
Here's how to get started with core cardio intervals using the tabata protocol:
5 minutes jump rope or any type of cardio to warm-up
Two of my favorite cardio core exercises are the updog snap and the alternating side plank. Click the tabata intervals link to watch a workout video that includes both exercises as well as the tabata protocol.