Chapter 9 and all of its challenges have treasure chests to destroy, so it does not matter which one you pick. I recommend the main level just because I personally find its the easiest.
Each section will has 6 or 8 chests to destroy. There is a objective list in the bottom left hand corner of the screen telling you how many there are and how many you have found. All you need to do is find all of them before you leave each section and at the end the achievement should pop.
Some chests are easy to spot and out in the open but others are very well hidden in the bushes and trees. There are also larger chests that are float in the water around the area you are in. It may take some good hunting but they are the places to look. To destroy the larger chests floating in the water you will need a gun, so if you find one don't use all its ammo until you have all of the chests.
If your achievement does not pop look at my solution for
The Giant pile of Gold achievement in Naughty Bear worth 155 pointsGot Gold Naughty Cups on episode 9 and all of its challenges. Yes, even the Top Hat Challenge!
There is a guide to follow if your achievement glitches.