Yes! Finally got Challenge 5. The tip I am going to give you is it only requires 1 Natural Gas plant and 7 flame turrets. A few Tesla towers helped as well.
The timer goes really slow on some of the last levels. :) I love the fact that you can choose the level. I started on level 1-5 then 7-9. :o I need to do 6 now.
"Then the zombies sang a catchy song, it went, "Trash Trash Trash Trash, Yum Yum Yum!", Then we fried them with our flame turrets, in the Super Energy Apocalypse." Ya'know, Lars, that could rival You Have To Burn The Rope. In fact, I can buy this on iTunes. It wins. :)
I find it funny that in this game America only make 20-40k revenue everyday. Think about taxes, funding from other countries and other such things. I still like the game, I just find it too, repetitive. I will still get the two hards, at least I want too.