A heat-seeking RPG really helps on this mission. Head to the yellow blips, take out the guards, head up the stairs, then follow the chopper, and bring it down to complete the mission.
Keep your distance at first, and use your sniper rifle if you have one to take out most of the bad guys. Once you're done with them, head down the ramp where 2 more bad guys are waiting - if you know they're there, you can take them stealthily. After the cutscene, kill the last guy as quickly as possible and get after that helicopter. Remember that until you climb the stairs, you have all the time in the world to lose the cops and regain health - head to the nearest fast food chain and order a salad or two.
The easiest and quickiest way to complete this mission is probably to go to the helipad, take out the five cops guarding the stairs onto the pad, followed by the two on the pad, pick up the RPG and AK-47 on the pad itself and then jump back onto the stairs and get on the bike by the entrance to the helipad.
You will probably have two wanted stars now so drive to the Pay-n-Spray near CJ's garage and remove the stars. Use the little ramp near the Pay-n-Spray to get onto the freeway (remember to not go nuts on the acceleration as you want to be in the right-hand lane not the left).
Drive as fast as you can to end of the freeway and at the T-junction get off the bike and wait for the chopper to come down the freeway. You should be able to get a few shots on it as it arrives (remember to lead your rockets) but then the chopper will hover over the T-junction allowing you to get three or four rockets in at point-blank range. This should destroy it.
- You can save yourself some chasing on the bike if you can destroy it as soon as the helicopter takes off. To do that, you need to equip yourself with some heat-seeking rockets (you can find some in the SF airport). As you walk up the stairs leading to the helipad, the helicopter will take off and fly from your right to go around the building to your far left. You have around 5 seconds to aim your rockets (wait for the reticle to turn red) and fire a bunch of rockets to it before it disappears behind that building. If you miss this you will have to do the usual bike chase.
- Cash: $18,000
- Respect: +50