It's is a bit tough especially the later levels. I'm rubbish at the game but our resident 12 year old expert (that has played games like Mario and Binding of Issac to death) is pretty good at it & has a high score in the 500s.
There are patterns in the way zombies spawn in the levels, once you have learnt these it becomes easier.
We have tested each level and each level is doable but they are tough on the later levels.