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Here's how I did it:

Kill as many bugs as you can in the first bug infestation at the beginning of the level.  Of course, ants first and save as many plants as possible.  When that's done, plant the cactus plants you start of with.  Get a spare shovel and water can.  What worked best was buying a few of the least expensive plants and then mostly buying the mid-level plants.  I didn't purchase anymore of the cactus plans as they were too expensive and didn't grow fast enough.  Try to plant at least 3/4 of the empty spots (if you can do more, that's excellent, but I still got expert without having every spot dug and planted, so do whatever you can!)

Also, buy some more of the bug sprayers -- I did okay with 4 or 5 or you can buy a couple of the overall bug killer spells and just stick to 1 or 2 sprayers.

The next bug infestation, kill as many as you can as quickly as you can, and try to keep as many color groups alive as possible.  I also went with storm cloud watering once I had a lot of plants rather than buying more watering cans. 

That was my strategy and it worked out all right.  If you have questions, do ask (or if another person has a good 4-10 strategy, I say "the more the merrier!").