Easy/Fast Treasure Chest Farming

First of all go to Gotham Diamond District, after go a bit north to the "Intergang Penthouse"

Once inside there is only 1 spawn point in this instance

After go to the Temple of Crime


There are 2 spawn points inside
the other one is on the final room (where you fight bruno) on the upper floor.

After go to the mysterious warehouse

2 spawn points inside

Only 1 instance left, the monarch playing card company

There are 3 spawn points inside


Just keep repeating this process and you will get the 1000 treasure chest feat in no time.
hope this was useful for you :)