You mean Sluagh? She's not impossible but you do need a lot of preparation. So it's gimmicky.
Preparation:
- Hero in Phantom Knight mode for Improved Ghost Party (entire tactic revolves around IGP)
- Jar of Honey. Bring a shitload. Buy cheap from Giant's Path smuggler.
- Yeon. You need a healer and her Rally trumps Felix's mass heal since it also buffs the party's magic defense. Sluagh packs some mean AoEs.
- OPTIONAL: 2-4 Potions of Brutality to disable Sluagh. She's resistant but not immune to Stun.
- OPTIONAL: Bottled Smoke. To Blind the zombies/skellies when IGP is down. Note: zombies/skellies IMMUNE to Daze
- OPTIONAL: Get Larelle some AoE attacks. I suggest Chain Lightning from Dr. Weird Vol. 3 (from Meline's quest) or Slime Wave from the Dragon Pearl (drop from Kobold random dungeon boss).
- OPTIONAL: Bronwyn. For her DoTs. Or just bring any DPS character.
Things to Remember:
- Have IGP up at all times until you whittle down adds to a manageable level. With this in mind never let Hero's TP drop below 50.
- Don't bother with Daze/Radiant attacks. The boss has a buff that makes her party highly resistant to radiant attacks and her mooks are immune to Daze.
- Key exploit: If you kill ALL the adds Sluagh will just summon all of them back. So kill all BUT one.
After saving the game use Potion of Brutality on Hero, Bronwyn and possibly the rest of the gang. Then trigger boss.
Turn 0:
- Get Hero's TP to 50 ASAP.
- Have the rest of the party throw Bottled Smoke to blind the zombies/skellies.
Turn 1:
- Activate IGP.
- Have Bronwyn Pin/Burning Pitch/Mana Shock the boss. She's immune to rest of Bronwyn's DoTs.
- Have Yeon heal when necessary.
- Have Larelle use non-Necrotic/Psychic attacks e.g. Lightning or Acid.
Turn 2+ (IGP up):
- Prioritize pumping Hero with Honey until it reaches 50. Prioritize also keeping party healed with Rally to buff MDF.
- Once the above are finished, whack Sluagh until she's Stunned.
- Keep Burning Pitch/Mana Shock on Sluagh at all times.
- Have Larelle keep on AoEing the adds.
- With extra TP above 50 have Hero activate Attract.
Turn 2+ (IGP down):
- Get IGP up ASAP.
- Throw bottled smoke on the adds.
- Use Yeon's Rally when possible.
Turn X+ (most adds dead):
- Once you whittle down the adds to preferably just 1 AND Sluagh is still alive DO NOT kill the remaining adds. DO NOT use any AoE attacks. If you do Sluagh will just resurrect ALL her adds to ruin your day.
- Once only one adds remain focus all attacks/debuffs on the boss. Sluagh is weak to Necrotic attacks.
- You can stop using IGP and honey and just tank with the Hero once only one adds remain.