I managed to complete this one on normal difficulty. I have also finished allot of the various mini games and unlocked all three of the Zen gardens, as well as the three of Wisdom or whatever it’s called.  I have managed to reach the night time pool level on the second difficulty. As far as I can tell, the difference in difficulty is that I get a few more zombies and Crazy Dave is picking three plants for me on every level. I find that if I adjust to what he picks for me, I can do pretty well.