The goal is to go as fast as possible with the 2nd priority of making big matches like bombs, stars, etc! When you make fast enough matches the upper left with start to fill up and as soon as you have the white words "Speed +(number)" filled in orange, the achievement will pop!
3-gem match = 100pts
4-gem match = 150pts
Took me a few tries until I finally got a board that gave me easy matches. Also if you need to reset, just hold your start button instead of backing all the way out. Your progress saves anyways and it's more time-efficient!