To get to the next level of the Deep Dungeon, you must find the hidden panel. To find it you must search all panels. Most likely the panel is at the opposite side at where you start.
The Deep Dungeon is the best place to find rare items so make sure all of your characters have the "Move Find Item" skill. Listed below each picture are cordinates where you can find an item. Make sure you save before every battle, if you don't find the item you want just reset and try again. Make sure you also have the catch ability because ninjas in the Deep Dungeon throw rare swords and items so you need to catch it.
X:0 Y:6--Elixer or Phoenix down
X:0 Y:9--Glacier Gun or Phoenix down
X:2 Y:2--Blaze Gun or Phoenix down
X:9 Y:4--Kiyomori or Phoenix down
X:0 Y:4--Elixer or Phoenix down
X:1 Y:8--Save the Queen or Phoenix down
X:7 Y:2--Blood Sword or Phoenix down
X:10 Y:6--Elixer or Phoenix down
X:0 Y:2--Elixer or Phoenix down
X:6 Y:7--Yoichi Bow or Phoenix down
X:9 Y:1--Mace of Zeus or Phoenix down
X:12 Y:5--Elixer or Phoenix down
X:4 Y:2--Faith Rod or Phoenix down
X:5 Y:0--Kaiser Plate or Phoenix down
X:5 Y:1--Fairy Harp or Phoenix down
X:12 Y:8--Elixer or Phoenix down
X:0 Y:0--Elixer or Phoenix down
X:0 Y:11--Iga Knife Plate or Phoenix down
X:2 Y:8--Excalibur or Phoenix down
X:6 Y:11--Elixer or Phoenix down
X:9 Y:8--Cursed Ring or Phoenix down
X:9 Y:9--Secret Clothes or Phoenix down
X:10 Y:8--Blast Gun or Phoenix down
X:10 Y:9--Elixer or Phoenix down
X:4 Y:4--Cursed Ring or Phoenix down
X:4 Y:10--Sage Staffor Phoenix down
X:4 Y:12--Koga Knife or Phoenix down
X:4 Y:14--Elixer or Phoenix down
X:2 Y:10--Elixer or Phoenix down
X:3 Y:8--Ragnarok or Phoenix down
X:6 Y:6--Robe of Lords or Phoenix down
X:8 Y:7--Perseus Bow or Phoenix down
X:1 Y:3--Elixer or Phoenix down
X:2 Y:0--Maximillion or Phoenix down
X:8 Y:4--Grand Helment or Phoenix down
X:8 Y:5--Venetian Plate or Phoenix down
X:10 Y:7--Chirijiraden or Phoenix down
X:8 Y:7--Chaos Blade or Phoenix down
X:8 Y:5--Elixer or Phoenix down
X:10 Y:5--Elixer or Phoenix down