Preparation For Babylonia

Preferable berserker since they do 1.5x damage instead of Nero and Tamamo since both females do only 1x damage

Last boss tiamat is a beast class boss so anyone will do since we also got permanent buff from merlin.
The only problem maybe Avenger Enkidu and Gorgon. He gain extra NP gain per turn.
Ushiwakamaru(corrupted) 1st battle is berserker class that will spam NP. Second battle against her she has no class + will summon extra ushiwakamaru per turn if there's less than 3 ushiwakamaru(corrupted) in field although only the original can summon clone.

Last Boss Niches: -
Tiamat Attacks One Target Per Action When She Crits, but attacks all allies normally.
Tiamat can Lock the NP of any party member.
Tiamat can debuff the whole team with her own normal attacks. The debuffs are Curse (3 turns each) and NP gain down (3 turns each)
Tiamat can buff herself with damage reduction buffs or Gain one tick of NP charge.
Tiamat's NP deals damage to all front-line party members and debuff them with Attack Damage Down, Defense Down (1 turn each) and Buff Negation (1 buff).
Tiamat can remove all buffs from the party, except the permanent ones.
But we also gain following buff: -

All party members gain the following buffs permanently for the duration of this battle:
Protection From The Netherworld : Attack Damage +50%, Damage Received -50%, NP Gain +50%, MAX HP +10,000

Chaos Tide : At the start of the quest and for every subsequent turn, the party's front row will receive 3000 damage at the beginning of the player's turn.
Wizard of Flowers : At the end of every turn, all party members will be healed and receive 4000hp.