Once you're level 18, red circles will show up on you minimap (not the big map). Enter it and it will say that you have to kill a member to start the fight. The red circles are based on time of day. Some appear around noon, and vanish around 20:00. Others are only available at night (20:00 to 04:00) (thanks to A Gamer X360 for this piece of info). As far I can remember, this gang attack was during daylight.
I recommend 'Boneyard', which is at the south-east of Sandy Shores. there is a perfect hideout for you right on top of a small building. NOT the warehouse building, but next to that, to the south of it. You'll have to push open a fence gate to get to it.
You can take cover up there from each side and you'll have a good view on the battlefield. You'll have to kill 15 enemies to finish the fight, and I killed 13 of them simply from that roof. There is no ladder to worry about, and they will only come from 1 or 2 sides (south-west).
The achievement popped after I killed all the gang members, not after I killed the 10th one.
I got this achievement on my very first try using the SMG with a scope and a supressor.
This is also a map for Survival, but there it is a bit harder with the helicopters.