This achievement should come naturally if you use your shield to kill the larger enemies. Play on Chilled difficulty and spread out your ships enough to give you a max multiplier in the final 2-3 stages. You can easily score anywhere from 30-50 million in the last 2 stages alone with a max multiplier
An alternative method is to play “Score Tilt” and speed up the game to give you a 10x multiplier. Using this method, you can get 30+ million very easily. If a 10x multiplier makes the game too fast for you, you can slow down the game until you get to enemies that give big points. Speed it up, kill them, and then slow it back down. Once you get to 30 million points, you need to complete that level.