1. Siren and 4 Titania. Siren can inflict poison with Diamond Dust and she may use it up to thrice per turn. The Titanias are less threat than Siren so try to AOE them at once in one turn before their ATK buff kicks in.
2. Ent and 4 Scylla. Ent has an AoE BB but the threat in this battle are the Scyllas. Kill them all at once as soon as possible before they can decimate your team.
During the Boss Battle, Wild Slash Bayley can use her single target Brave Burst Scratch Shot or AOE Brave Burst Scratch Strike which she uses every four turns. Both can poisons your team, and can attack four times in one turn. She can be weakened, paralyzed and injured.
Nyan Slash Bayley casts an AOE BB every 4 turns that can cause Poison. Very dangerous as it has ability to reduce character health by around 2000-3000 and she may continue attacking afterwards. Guard just in case to avoid death.
Fire-only squads are not recommended due to the high appearance rate of strong water units.
If you do decide to bring a fire-only squad, placing a Drakeborn Lodin or Drake God Lodin in your squad is an effective way of fighting Sergio and Siren.Also if you have or know a friend that has Gemini Il & Mina to avoid being poisoned and to save some item space. Blaze Sibyl Freya and Ray Sibyl Sola are also an excellent unit to use in these stages as they can attack with multiple elements.
Bayley can cast her BB, Scratch Shot or Claw Strike (Lv. 1), Slash Strike (Lv. 2), Scratch Strike (Lv. 3). All have the potential to cause Poison. Bring Antidotes.
Red Bone, Blue Bug, Green Bug, Red Bug, Red Claw, Blue Drop, Green Drop, Red Drop, Earth Eye, Fire Eye, Light Eye, Thunder Eye, Water Eye, Green Fang, Theo Feather, Blue Grass, Green Grass, Red Grass, Hoku Nut, Koka Nut, Giant Pelt, Red Pelt, Paladin Stone, Fujin's Eye