You'll need:

• An AoE tanking unit
• Status Break resist
• MP draining skill
• MP recovery (preferably instant, not refresh)
• 40% ATK/MAG Break
• Confusion resist
• Good defensive buffs

Highly recommended:

• Mechanical Heart for your tank
• Soleil or Zargabaath's buffs
• Damage Reduction Buffs
• Pod 153's Physical Damage Reduction
• Light or neutral damage
• Illusionist Nichol 100% evade unit

Recommended units:

• Warrior of Light - AoE Physical Cover and 45% ATK/MAG breaks. Warrior's Pride procs after the 50% threshold will keep him alive easily.
• Veritas of the Earth - AoE Physical Cover, Status Break resist, Damage Reduction buff. Lots of counters for the Summon Titan & Lakshmi mission.
• Charlotte - AoE Physical Cover, MP refresh. She can also resist Fatal Attacks consistently when enhanced, though this may not be that useful.
• Luka - Ability heals (No White Magic mission) and Status Ailment cleanses every turn.
• Tilith - Full Heals, Status Ailment cleanses, MP battery, Status Break resist. Ability Heals.
• Ilias - Status Break resist, Confusion resist, MP battery. Use his Salve 'feature' or his own MP recovery skill.
• Soleil - 120% buffs, enough said. She can also reach 100% evade easily.
• Rikku - 30% Damage Reduction and AoE reraise will ease your tank's work. Can reach 100% evade too.
• Zargabaath - 60% Buffs and Confuse resist, Damage Reduction buff and 100% HP/50% MP recovery. MP draining and plenty counters for the Espers mission. Elemental Resist buff if you can fit it in the rotation, too.
• Ace - Best MP battery, Light Imperil.
• 9S - 50% Breaks, Damage Reduction, his TMR is highly recommended.
• Marie - Elemental Resist, Supplementary Heals, Confuse Resist.
• Minfilia - Discount Marie.
• Ling - MP battery, 45% Breaks. Can reach 100% evade.
• Eve Illusionist Nichol - His LB is pretty useful here, 45% ATK/MAG break and he can turn a 100% evade unit into an AoE cover tank.
• Orlandeau - Light damage, MP drain, 40% ATK/MAG Breaks.
• Veritas of the Dark - Chains with Orlandeau with a Light weapon, 45% Breaks, Damage Reduction buff, Dark Resistance. MP drainer with Osmose Blade or Lance from Odin.

• Eve Grim Lord Sakura - She was basically made for this fight. AoE Status Break resist and Confuse resist in one ability, 100% DEF/SPR and 50% fire resist buff, immune to Confuse herself. Neutral damage. MP drainer with Osmose from Diabolos/Ramuh.

• [No Icon] White Witch Fina - Fully enhanced, as an MP battery. Her high innate SPR will keep her with enough MP to use Ritual. It will not last for long unless you use some form of Refresh on her, as it does not heal herself with it. You can wait for the next week for a chance of pulling her if you didn't get her last Halloween.

Tips:

• First of all, unless you're doing the 100% evade unit iNichol combo, your AoE cover tank will need at least 7-8k HP and 400 DEF to endure all that damage. Mechanical Heart will really ease the need of healing a lot. Also it's not worth it to do the 5-man mission, for real, it's not, avoid trying that.
• Status Break resist, ATK/MAG break, AoE cover and some sort of defensive buff MUST be applied on the first turn. Otherwise you're pretty fucked. This will also need to be applied AT ALL TIMES.
• If you don't have Soleil, Zargabaath or Grim Lord Sakura's defensive buffs, you'll need to add Damage Reduction buffs on top of that. Even better if you stack Physical Damage Reduction (Pod 153).
• First priority after that is draining all his MP as fast as possible. It's better if your Damage Dealers are the one doing so. For physical, equip Osmose Blade or Odin with Lance. For magical, equip Ramuh or Diabolos for Osmose.
• Normal attack for the next turns after that to grind those Esper Orbs.
• Confusion resistance is really not needed until later in the fight. Try to fit it in your rotation when your unit is not really busy doing other more important things. The esper Lakshmi has an skill to protect against confusion.
• After you summon Titan & Lakshmi, start going ham on him.
• When you reach the 60% threshold, he'll start doing AoE Osmose. You'll need your MP battery for that.

• You'll need to Osmose him again every 2 or so turns (depending on your average SPR, it may be every turn for you), as his regained MP will make his MP-scaling skills start doing damage again.

• Edit: Some people are reporting being drained fully every turn. Try either using Magic Damage Dealers (higher innate SPR and MP pool) or stacking your MP battery with all the SPR you've got.

Of course, you can do this with any other Damage Dealers, these are just the most suitable for the fight. Most important thing is they must carry any form of Osmose to dry the boss out of MP.

If you followed these tips (or not) and beat it, congrats! Enjoy your new Serpent Mace!

For those curious, my team consisted of:

My Orlandeau could have easily been replaced by Agrias for the full 4 stars unit run.