The Light and Dark Cave presents a pretty good challenge.
Basically, there are two types of stairs in this dungeon - light and dark. Only one type is open at a time, and in order to change between the two, you need to activate different lanterns. (Basically, when the lanterns are unlit, Dark stairs will be open, and when they're lit, Light stairs will be open.)
When you enter the cave, the lantern next to stairwell A is unlit. Light this lantern, and take stairs B to get treasure T2. Then go back up to the top floor, get treasure T3, then go to the northeast to take stairs C.
Un-light the lantern in the northwest corner, then take stairs D to get treasure T4. Now go back up, re-light the lantern, and continue on to stairs E. Now go south to stairs F, and then go north to challenge the boss of this dungeon.
I used Blont, Bruce, Linn, Claw and Temi for this battle. (Though, honestly it doesn't matter who you use as long as you have a good strategy.)
I had my fighters attack every turn, and Temi use Cure as needed.
Cavern Demon doesn't seem to have any special attacks or magic, so it's not a hard battle; it just takes awhile.
Anyway, after the battle, there's still more treasure to be had. Grab treasure T5, then continue to the north and take stairs G.
Now go all the way to the south to stairs H. Get the treasure there, then go back towards stairs G - but un-light the lantern along this corridor. Take stairs I to get treasure T7.
Now, leaving the lantern unlit, go back to stairs G, and backtrack all the way to stairs F. Go north to stairs J, and open treasure T8.
Now warp to the entrance and talk to the mysterious man to receive a Black Gem. Now it's back to the Empire!