You will need a strong weapon: M4, AK-47, sniper rifle and a lot of clips to take down a helicopter. Or a rocket launcher. If you don't know where to find those lying around in the world, you can also use the weapons cheat:
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Then get to an position, where the cops can't ambush or close in on you. Start a rampage killing civilians until you get 4 wanted stars. After a short time, you will hear a helicopter (respectively the pilot yelling at you). Take it down. It takes about 1-2 minutes until another helicopter spawns. After the fifth one, the trophy pops.
If you don't want to cheat: You will get a rocket launcher (and much other stuff) in front of your hideouts, when you collect all the hidden packackes. As suggested by Kornfan2007
And, as always: Before you downvote this solution, please write down a comment first and tell me what might be the problem. Random downvotes are uncool. Thx.