I got a strategy for impossile (i did beat it on impossible). The zombies arrive in the city by coming in from the outside tiles; those tiles you cant enter so theyre basically not a prt of the city. It may seem like the zombiem population is randomized for the tiles you can claim but you need to imagine it more like a lowered plateau with water flowing down into the plateau: Its gonna first fill out the whole plateau in a very shallow level and then continually rise starting with the outter part of the plateau.
Same with the city youre reclaiming: The zombies will heavily infest the outter tiles of the city because these they will reach first and then spread out through the city. Thats why if you do a kill mission on one tile and then leave it without reclaiming, the zombies from surrounding tiles will enter it again. That said, some special tiles like the heliport, the big graveyard or the city hall are set to have a large zombie population.
Now to the actual strategy: The main goal of this strategy is to stop the inflow of the zombies from the outside, as well as stop the Horde Events which also approach from outside. In order to do that you need to establish a border of reclaimed tiles along the outter reach of the city. On impossible this is a pretty hard task and you will lose many tiles: The four starting tiles should be defended for some time but their loss is both inevitable and unimportant. Whats important is that you first push for the outer Edge in a 2-tile line of reclaimed tiles and then start reclaiming the "border"- tiles. When you just arrive at the border your state of affairs should be pretty bad so just do it one tile ata time. Soon when youve recovered enough do it two tiles at a time, by going clockwise and counterclockwise. As soon as youve reclaimed the last tile you have inoffically won. From now on, as long as you dont lose any of the border tiles, the enemy only has one certain number of zombiees it can throw at you and every single zombie killed is a definite casualty. In my save with this strategy implemented i can just click on every unreclaimed tile and see that It´s "safe-ish" so the need to reclaim that tile isnt there except if i want the tile bonuses.
I hope you like my strategy and can successfuly apply it as I did