If you are aiming for the Sharp Shooter achievement you just have to follow two simple steps.
Where to Get the Achievement
You just have to enter the city and go to the right and inside the building.
How to Play and Get the Achievement
You have to pay 30 gold to play, you'll get rewarded some special money to buy stuff at your left.
You have 20 shots and must hit ballons, but not manure. It may be hard if you don't have machine-like reflex and an absolute accuracy.
Fortunately you don't have to get 100% shots, only 90%, that's still 18 hits.
You can retry again and again and you may succeed but there's a little shortcut here.
If you put your crosshair on the crack in the wall and just move up and down you can hit a lot more often, because of the movement of the balloons, almost all of them wiggle from left to right, and at this exact spot the movement is going left at the opposite of the general direction of the balloon, so it stand still for half a second.
Take your time and take the shot if you are sure of yourself, no need to try and get'em all, maybe avoid the ones that moves a lot, oh and your score doesn't matter, ony the accuracy give the max reward, if you don't hit s**t.
Of course there is others spots where this happen, but this one is easy to spot and remember.
That's all I can say, be patient, it took me 15 minutes. Good luck!