Here is a great short set that really makes your legs tired by demanding full power out of your dolphin kick.
10 X 25 with fins
Odd 25s are dolphin kick. FAST under or above water with arms in streamline.
Even 25s are dolphin kick on your back. Keep your arms at your sides and position your head so that you are looking at your feet. Watch your feel as they come out of the water on each kick. These kicks should be VERY powerful with a large swell of water surrounding your feet.