For all you guys playing JP, I just muddled through Expert (上級
in the Dark Mech Dragon of Destruction dungeon.
Devil Dragon (level: teens), Keeper of Light, Rainbow Crystal, Thor (level: teens), Echinda (level 43), Archangel. (level 30ish)
Most of my damage on the bosses came from Rainbow Crystal where I'd let him knock me down to about 80 hit points and then hit him for like 70,000 damage, before healing back up. It's a longer run, the boss fight taking about 120 turns total (4 Crystal Rainbow shots) but know I can I run it, so I'll be doing this for awhile.
I've read in user comments on the JP wiki that the drop chance in expert is probably around 5%.
Oh lordy....