I managed to do this on the fourth level in Action Mode.
Just let the time bar go down a bit and then keep doing matches and score points, making the bar go up - but be careful, if the time bar goes all the way up, you will continue to the next level.
When you are sure that you have scored more than 500,000 points in the current level, just fill the bar to continue.
You MUST complete the level to gain the achievement. I became a bit confused about this achievement when I didn't get it in level eight or so with a score of ~700,000 in that level. The problem was that i didn't COMPLETE the level.
Negative vote? Let me know why.