You can only do this if you are willing to cheat, and from the sounds of it you are.
Go on the beach with a mini gatlin gun and start ripping people up, put in the lower wanted level cheat whenever you need to, the people should drop a lot of money. Other guns work too.
Instead of doing that... here's something better with no cheating required: go get a Sparrow or Sea Sparrow and go to the beach and just fly it along the beach tilting the blade down and chopping people up... the Sparrows fly so that they don't flip so when you do this there is no need to adjust and no fear of damaging the helicopter... plus... you get no raise in wanted level if you do this... even if you kill cops.
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