Just beat BL expert 15!
I don't know how much help I can be, but if anyone has any questions, feel free to ask.
My primary lineup was consistent, but I changed some moves and talents (noted in parentheses):
Angelo (Unnerve BL 1-12) (Intimidate BL 12-15) - Psychic Thrust, Absorption, Mind Bending +2, Water Bombs +2
Devil (Unnerve BL 1-12) (Intimidate BL 12-15) - Hypnosis, Absorption, Boulder +2, Cunning +2
Leviathan (Thorn II) - POD, Water Torrent, Rain Prayer (BL 1-10), Water Bombs (BL 11-15), Absorption
Neon (Adaptability) - Thunderstorm, Lightning Strike, Electric Net (BL 1-10), Psychic Thrust (BL 11-15), Slash.
My sub:
LVL 50 Brawny - learned techs only. I started building him up in preparation for synthing Setanic. Anytime I couldn't use one of the monsters above, I subbed him in. As a lvl 50 with no great moves or stat bets, he wasn't overly useful, but was pretty stout situationally. Since there are a number of epic and legendary fire monsters in the last levels, Brawny could often take one out or at least buy time for someone else to heal. He was especially useful in softening up opposing Cerberuses when my stronger monsters had been weakened.
I actually picked up Leviathan during Holy Ruins expert mode (finally got enough ladder prestige to open some of the unique monsters for silver). Levi was the MVP most of the maps, but Angelo and Devil were extremely useful. Having multiple monsters than can heal themselves seems like a must by the later levels, since items are often locked and you'll need to deal with much higher than average misses with your skills. All my main group was level 60 by the end, though Levi was highly useful as early as level 50 with POD.