If you are stuck on battle 8, try using the shine apples. Build two on your tree immediately the game starts, and nothing else. They will kill about half the wave, letting some bugs double up to come back at you. You earn more water points than it costs to keep your tree healthy, so you can keep on doing this over and over with the same wave. Sell the apples if you need to let more through.
By the time I get to the second wave my tree is full size and I have 200,000 water points saved up. You only need level 1 apples, and if you are short on points to buy the skill in the first place you can replay the earlier levels, again, keep letting some bugs through to multiply so you get enormous scores.
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