my strat if I remember it correctly:
the Lustlord Guardian have [no status weaknesses, and resists Fire, but is vulnerable to Ice, Earth, Sex, and Anti-Sex. It has a magic attack that poisons all, a magic attack that does physical Earth damage, and a strong single-target attack].
At least put the first party member that can deal Ice, Earth, Sex and Anti-Sex on each of the 5 group.
Then put someone that can heal or tank the damage so the group doesnt get wiped in 1 critical attack.
Then its just filling in the leftover party member to what group need additional firepower or defensive skill.
You have 14 party member to be divided into 5 battle group.
Since 4 of them can be mix and match with 3 member, the last group will only have 2 party member.
I remember I filled it with Nalili and 1 healer (either Uyae or some other mage, since Carina is better suited to a 3 member group for her group Defensive Shield skill).
Nalili had sex-element skill and have good ATK stats to just dish out normal attack. Coupled with Combat Masturbation, her stat can be pretty monster.
The healer then just have to keep her health from reaching 0.
Hope this helps.