's solution is short, sweet, and straight to the point. I'd just like to add that this can be done on any difficulty (preferably easy of course), and also share this video with you guys that I have put together.
Starting off from level 1-1, the next 25 destroyable buidlings towards this achievement will take you all the way to the beginning of level 1-4. There are no buildings you can destroy in level 1-2 so just charge through the level without backing out.
Check out the video to see exactly how it's done, and enjoy!