Shoot them in the back.

I've found that the trick is to attack the creeps as they recede from the laser towers, rather than as they approach. Place towers slightly above level with the point of each bend. See Photoshopped red lines below.

I spent many hours trying this level before I happened on this solution. I also upgrade the first two towers to level 3 before 7 starts and leave the rest alone.

From there it's a matter of spending the cash you get fast enough. More towers and more upgrades, preferably at the bottom. I'll post pictures of the end game if needed, but this "shoot them in the back" notion was all I needed. I beat it on the first try after that.