You earn DNA points while playing through the levels destroying objects and defeating enemies. When you have collected 1,000,000 points the achievement will unlock. This counts as the total collected throughout the game and does not mean that you have to have a million points on hand all at the same time. With that in mind, feel free to spend the points as you get them to max out your aliens. This will require completing the game and then replaying levels to get the total amount OR see "100 Hit Combo" for a relatively quick way to 1,000,000 using Swampfire on level 2.