Actually, what i did, run up as fast as you can to the very top to the helipad. I encountered about 3 going up there. Then from there on just camp on the helipad for them to all come to you. You'll see when some come through the fence to your left. Flashbangs come in handy for the ones that come in 3 at a time. Use the 50 Cal and the MG240 or whatever it is