To get the hard achievement pop star: Just keep doing the other achievements, until you've got 500 monkey money. Then, just start pretty much any expert track on hard, get the double money power up and put a couple ninja monkeys on the beggining... Then, just get one or two bomb towers and a spike factory near the end of the track... after upgrading them, just keep buying super monkeys (and upgrading them to robots) and maybe a few banana farms if you can.
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