This really isnt as hard as some people a lot of people are reviewing this game as- you really capture the aspect of survival. However, once you get the minigun and are fully upgraded and start stockpiling money, its all about remembering to buy health packs. An armor upgrade would be pleasant.
I liked how i could almost use strategy in terms of different weapons/situations, such as using the flamethrower for dense groups of big zombies, and the minigun for more spread out zombies or birds. Birds arent too bad if you use minigun.