Keep going back and forth upgrading your lasers. Eventually, you will want to add a laser at the top points of each Y shooting down at the backs of the creeps as they go by, and you'll need to upgrade those a bit as well.
By the end your original lasers should be at level 7, and your top lasers should be at at 3 or 4 or so. Before the last wave, sell everything on the left side of the map, and just start spamming missile towers on the right, getting as many down and upgraded as you can. Keep doing this as you get more money.