I would add the Piercing Ice Shard to the spells that could help, great freezing chances and, as the name suggest, it pierces through the ennemies.
I made the boss with my party as follow (your members should engage the fight far from each other to avoid a total annihilation from the Meteorite skill) :
My Scoundrel (Lone Wolf so maybe you will need 2 members here):
Started on the right side of the room to handle the Poison-ghoul right off the bat
My Wizard:
Engaged the fight, chaining Piercing Ice Shard to hit both the boss and the Fire-mountain thing
Madora :
Started on the left side of the room to tank the Bone-thing through the fight
As stated above : Rain is your friend here. The goal is to kill both the Poison and Fire bosses ASAP and chain on the boss.
Hope that help a little, good luck !
