In addition to what tsunamishadow wrote, one thing to note is that you can get this achievement when doing a fitness routine. Anytime you can kill 2+ birds with one stone, you might as well.
For those wondering, I got the achievement doing the 35 minute routine (I think it's called 'Intervals'). I think I got the gold star on "Bulletproof". And...I did the routine on easy.
Since I hadn't done anything related to fitness mode prior, I got 3 other achievements during the routine: completing a playlist (pops upon completion of routine), completing a fitness playlist of at least 15 minutes (pops upon completion of routine), and doing a playlist of at least 30 minutes (pop mid-routine).