Upgrade lifespan until level 4; no more. It’s useless due to the bad zombie programming; you can have 50 zombies with max lifespan in one side of the map and they will never kill the last survivor at the other side.
Then focus on infection rate; a single zombie can turn into a huge army if you have that upgrade at a high level.
Once you destroy some towns, start focusing on speed, attack and resistance, while keeping infection rate as high as possible.
Buy extra infection shots as soon as possible. Also upgrade the directions as early as you can.
Even if you have to grind for some minutes, it’s worth saving for both penetration upgrades early during the game.
During combats, don’t buy support unless the scenario asks you to. They’re useless.
Same for heroes, don’t unlock them until you need them to destroy a town. Don’t waste money upgrading them.
I used the following heroes:
Jigsaw, Jason, Freddy & Ronald.
Start by using your shots (or support if the scenario requires you to do so), then spam your heroes in the most defended area or next to the enemy hero, if any.
If multiple areas are well defended, spread heroes accordingly.
If a scenario is too hard for you, just skip it and keep going. Try it later, once you’ve got more upgrades and you’ll beat it without problems.
Following the above tips, I finished the game in about an hour (got the 60 minutes achievement whene there were two towns left).
Then I continued playing, there’s one town missing, but I’m done, since I’m missing its time trial which is totally random and that particular level lags a lot. not worth wasting more time in this game.