Must be one of my favourite flash games ever.
Strat for relatively easy win inspired by a comment below:
first, never stay on the car, you wanna repair it as soon as possible and if it gets damaged you waste time healing it. You are generally agile and tanky enough to be able to survive on foot (as long as you dont just run straight through a hoard of zombies of course, keep jumping), and in a pinch you can buy a medkit from the store. Also you can drop the car parts by pressing down again in case youre carrying one but theres an enemy approaching. As said, you want to repair the car asap so always press down when possible, inbetween reloads or if you happen to be near the car and theres no enemies immediatly next to you. Repair stops if you move or shoot, but reloading seems to be fine (also you can reload on a full clip which is a bit annoying).
In terms of skills: as far as i can tell car repair speed only applies to healing its health so its useless as it shouldnt be damaged much anyway, same for car health. Accuracy is also useless with any weapon beyond your starter pistol. I recommend for your first 2 skill points to go into reload speed and quick learner, then a point of extra cash and a point of regen, then try to max out reload speed ASAP, then max money, then recommend to get a second point of regen for the end.
For the shop: ignore the revolver and save for the winchester, then ignore everything else and save for the M16, after that you can do what you want with your money. Personally didnt need to use mines but they could be very good for clearing out crowds iirc.
Using this i finished right before wave 13 (turned to wave 13 while the car was leaving)