Sun & Moon Temple Back Floor
The Sun & Moon Temple takes us back to the traditional back floor method where you must find an item that drops from within the dungeon and use that on the coffin to get to the back floor. The item you're after in the Sun & Moon Temple is a Secret Path Key. As seen in the screenshot below.
Using this item by the large ? on each floor of the Sun & Moon Temple will activate the coffin and grant you access to the back floor. That's all there is to it! I recommend using Xiao mostly for this dungeon as a lot of mobs can get off some lucky close range hits which beat up Toan pretty quick.