A bit too easy, you could apply the same solution to every level.
1. upgrade gold income
2. spam shovel dudes until you get enough time to upgrade to 10 max farmers
3. repeat until you can afford 10 farmers at once, then send those and wait for the next income upgrade
4. repeat 3 until gold is maxed
5. get max farmers to 30
6. get shotgun dudes
7. send a mob and get 15 shotgun dudes
8. either continue getting more shotgun dudes, or get tons of survivalists
9. no zombie can reach your ranged barrage!
Try clearing your cache and cookies, then try again. I got that same problem a while back, but it disappeared after I used CCleaner (which clears the cache and cookies).
Best game ever! Although quite laggy and buggy, but that only adds to the chaos ^^ Achievements and clothes other than glasses/hats would be sweet.