Easiest way I found was to drive in through the barrier, absolutely do not stop near any soldiers, approach the attack helicopter with your vehicle and sticky bomb it as you drive by to prevent it coming to shoot you down after you take the cargobob. Assuming its the escape you're having trouble with.
Now the Shuai-Jan, is a snake that is found in the Ch'ang mountains. Wait, I know you.
Get a car, drive in from the south or south-east. Drive straight through (not past) the soldiers to avoid being ripped apart by machine gun fire. Drive up to the cargobob, get in and quickly turn 180 degrees and fly away. Land on the mansion with a flat roof on top of the first hill you reach, get out and get your anti material sniper rifle and shoot the attack helicopter (be quick, otherwise you'll be in range of his rockets). Easy.
I ramped an SUV over the fence as a sort of shortcut. I went straight for the helicopter and never fired a shot. By the time they sent a chopper after me, I was already far enough away to easily lose it.