Level 20 is done with the best beam weapon around, and just dodge around the edges, do one special for the AMU, then save the second for the 3 turrets, as soon as they spawn, you can snipe out one and damage the other one, and then you should be able to polish off the level.
Tiers are backward.
I'm laughing at my result.
...He will always remember the intensely boring days spent counting money. He feels bitter that too much of it was spent working.
...Amazed at the effort given for education. It is clear to everyone he will become a virtuous priest.
lulwhat?
The bridge doesn't need to be that strong, connect it to the two points on the top, and the one at the top right of the shack side. I finished it with $1868 left over...
Aha. The problem is that you have to do the levels in ORDER for them to count. Thus, you can't do any level and count it before level 1, nor can you do any challenge other than challenge 1 at first.
5th challenge I ran off of wind power and flame turrets. I had to restart the 1st time, because the vehicle zombies spawned in the 1st two waves, when smog was high enough to one shot my oil, and so I couldn't fire back the rest of the night.
5th is all about not using natural gas plants until smog is less than 200 (maybe even 100).
I did it on super easy, but it seems there is no easy way. On every single night I ran out of energy (3rd night was just bad luck) and had to be saved by daybreak.
 "Higher!"){kind=icon}
 "Cube Colossus"){kind=icon}
 "Madness interactive - Next-Gen mod"){kind=icon}
 "Dead Frontier: Outbreak"){kind=icon}
 "My Pet Protector"){kind=icon}
 "Little Wheel"){kind=icon}
 "Cargo Bridge"){kind=icon}
 "Battalion: Skirmish"){kind=icon}
 "Super Energy Apocalypse : RECYCLED!"){kind=icon}
 "Dodge"){kind=icon}