I was having a similar experience. I kept strategy in mind and played for somewhere between 2 1/2 to 3 hours, but so many sniper dudes kept spawning that it was impossible to avoid and once they were resurected my group was instantly swarmed no matter what i did. sometimes i could kill one or two waves, but never with a profit and i never made it past 85 zombies. i would recommend at least toning down the sniper spawn rates because it gets near impossible to not run into a group with at least 2 in later stages, or maybe turn down there frequency to fire, or limit there range, because i would often find myself having the sniper hit someone across the map in a big group, that group would then push in other groups and i would be wrecked in less than ten seconds when i had been playing for 5- ten minutes. the speed and agro timer also make it near impossible to escape and try to recoup losses and you cant even move fast enough to escape the other groups.
while i see a lot of potential here, the game is nearly unbeatable in its current state and even trying to recover is impossible, meaning you'll always end up starting from scratch when it should be unnecessary. hope you the best and good wishes towards the future of your game.