If you've done Phil Cassidy's missions, go and buy the M60 or buy an M4 from the Downtown Ammu-Nation before the mission starts. I'd really advise an M60 because its bullets are stronger than the M4. Start by shooting the Angels at the front of the club and even shoot the guy in front of it. Continue to shoot people and cops and aim for a 3-star wanted level. When the helicopter comes, you can use an M4 or M60 to shoot it and for every about 10-20 bullets, the meter will start to fill in. I wouldn't advise using a rocket launcher because, even though I haven't tried it, it blows the helicopter too fast and you'll have to wait for another to show up.
When you complete the mission, you'll still have the wanted level regardless if you pass or fail. Quickly save in the Shomole Shack and reload to clear the wanted level.