Here is what I'm doing:

-choose the level you want to export;
-select all parts of the level (CTRL A );
-from the "3D View" menu choose "Export to OBJ format..."

I know Pruse slicer can open .obj file - you will have to re-size it to a value like 7% (I guess you print a small to medium design)

Repeat the above steps for each levels of your design