I also find it rather interesting that the Spitfire burns enemies, but doesn't light them on fire and spread the heat of love... They're flame retardant, but not immune to burns?
It's amazing how I can run around shooting zombies, collecting cash from dead hitch hikers, and burning up my limitless supply of car fuel to burn down zombies. Amazing science. Anyways, tips: If you build on accuracy, reload, and regeneration, you should be good for survival for the later waves if you run back and forth and jump over the zombies. It keeps you alive, and it keeps the zombies dead. For cash, obviously build your businessman skill so you're getting some decent cash. Late in the game, if you're running out of reasons to spend your money, just buy a huge load of mines and lay them down all over the place. In later waves, experience isn't hard to get at all, since they send hundreds of zombies at you at a time, and the larger the zombie, the more experience it gives. Of course, kill the smaller zombies first, since they're fast, and save the really big zombies for last and just jump over them to avoid them. Good game, 5/5!
I've noticed that if you create a network of relays connecting directly to any of the areas you'll have weapons, it helps to distribute the ammo packs faster.
If you're going through as a warrior, my suggestion is to leave discipline alone and work solely on getting every passive in the combat and rage areas. These will make you quite formidable, and with impale, you'll be pretty much safe for most of the game.
If it'sa billiamom, then that's one unhappy mother. seriously, she's probably thinking "why do all of my children go off to join the ranks of turtles that have been shot out of tanks and into spiked walls... *sigh*"
HUGE balance update coming! All strategies may be turned upside down!