easiest way i found to get this was to play the game on serious difficulty at 25% speed, just stay at the top of the screen and await 2 enemy ships to arrive from then on according to the enemy just move top middle or bottom, and tap screen for shield to explode.
i managed to get this this way on the second level all i did was use level one for practise and roughly counted along as to how many i had destroyed i was very generous with my counting so it might be possible to achieve this on level 1.
hope this helps guys.