^ Great source of information to get an idea where to find them.
I can not confirm but I believe if other people open chests(In successful missions) it counts for you as well. I hardly open chests and it randomly popped for me the other day in a mission I was playing with others.
I'd also like to recommend if you are going for all the achievements to not worry about farming this specific one until you are done with Twine peaks because I guarantee you will have it before you're done(I'm mid canny and got it).
If you are, low level fight the storms are great(Don't go for grassland/forest missions). Build a pyramid around the atlas, place a defender pad(with defender of course) and scout the area. I'd recommend either a ninja(any) for speed/double jump or Outlander(preferably eagle eye) to detect chests(have to be "in the zone"(smash a few things)). 7 minutes should be enough time for you to find a couple chests.