Sounds easy right?
Well like all the achievements in this dlc, they are a bit buggy.
After shooting the trash on the left side, hitting two clan guys i could never get the achievement to unlock.
The trick i found was that it seems to only work if you shoot the clan in a certain order.
Start by shooting the guy up on the far right, then the two on the balcony and finally the two below them.
You should now be left with two clan members next to the cart of garbage. ( the two closest to you )
Now aim for the very far right explosive on top of the cart which will make the whole cart blow up and take the two guys out.
This wont eliminate all of the clan as 4 more will then spawn but it appeared to be the only way i could get the achievement to unlock.