Beating the expert dungeon is easy as long as you prep correctly.
The team I used was
weakest blue orge + Sapphilit + Sapphilit + blue demon mask + blue demon mask + siren3 helper. Its all about having low hp.
Its really important that you have 6 of the same element since it will kill a divine mask in 3 attacks while a 5 of the same element team needs 4. If you want you can make the green version of the party. Having a healer be able to 70+ percent health heal is essential too.
The name of the game is prep so this could take a while.
Always take out all but one enemy obviously take out any enemies with the same counter first.
Prep so you have this for the next fight (except for the last fight.) Never know when the game refuses to give you a blue.
B = Blue
M = other than blue
MMMMMM
MMMMMM
MMBMMB
BBMBBM
BBMBBM
For the last fight go.
MMMMMM
BBMBBM
MMBMMB
BBMBBM
BBMBBM
You don't need to have exactly this but more blues the better. Also make sure there's good variation in the Ms so you don't accidently combo.
You'll have 4-5 turns after this and most of the time you only need 3 to win the fight. Fought this twice and haven't had any issues(No Mystic Mask Though
)