On level 70 you need to destroy the crates at the bottom to open up the portals as they will help you fill up the upper area with pieces. It is quite possible that you might not get any matches there though as the space is quite limited. In this case, simply focus on getting a Lightning power-up and use it on the same colored pieces as the pieces in that area. Combo explosions will get you a Lightning in no time, and just two or three Lightnings guarantee your victory!
Another effective method of winning this level is to make a power-up (even a firecracker might work, but we recommend getting at least a bomb) on top of the portals and then using a Lightning to make the power-up fall into the upper part. It takes a bit of practice to maneuver pieces like that, but we are sure you will succeed!
You should also check out this video that shows how we got through level 70.