Main Walkthrough

Location: Solitary Cloud Marsh Pier
Prerequisite: Complete Battle with the Beggars' Sect, begin Trip to Nanjiang
Petitioner: None
Reward: None

In order to trigger this quest you need to complete Battle with the Beggars' Sect, which is the 'good' ending to that line of quests. You also need to reach the main story quest Trip to Nanjiang, which allows you to access the southwestern, jungle-like section of the world map (conveniently named Nanjiang).

Travel to the western half of Solitary Cloud Marsh Pier, on the Nanjiang side. You can do this either by traveling there via Jinglong City, or by using the Boatman at the south of Solitary Cloud Marsh Pier's eastern half. Either way, you'll hit a cut scene when you walk just about anywhere in this small area, and it leads to a battle. 

(I highly recommend removing Li Yuanxing from your deployed characters before getting into this fight. He'll push his way into the battle anyway, giving you an extra party member for the fight. You can use all the help you can get for this one.)

The Villains

Woof. You probably haven't seen a fight this large in a while. You're facing off against a sizable contingent of baddies from Villain Valley, and with perhaps one exception they're all quite powerful. Their ranks consist of the following opponents:
  • Sha Quangu, a polearm user who uses a lot of AOE attacks
  • Du Biao, the Gusu City creep with hidden weapons who will often Poison characters
  • Bao Min, the hefty bruiser with the polearm who also likes AOE attacks
  • Night Owl, the weakling of the group who favors hidden weapon attacks
  • Villain Valley Disciples who like sabers
Included in your party for this battle are Xun Yaoyao and Yan Weihuan, who are unfortunately stuck on the opposite side of the battlefield at the start. They control themselves, so take advantage of their durability to keep half of the enemies occupied. They won't be in great shape by the time you break through the first wave, but there's not much you can do about it.

Otherwise, form a wall against the villains and target their numbers with AOE attacks as often as you can. Keep your ranged characters back a space or two so they won't get hit by AOEs. Favor the named enemies whenever possible, as they will do much more damage than the lackeys. Your characters will get Incapacitated often in this battle, so don't expect to move around all that often. Push forward slowly and you will, after a while, take out all of the baddies.

You'll receive the Big Dipper Boxing Gloves Advanced Blueprints, the Madness Staff Technique manual, and a good piece of equipment for winning this battle. (I received Black Gauze Heavenward Boots.) Xun Yaoyao and Yan Weihuan will take their leave...

... but you can find them more or less immediately. Head to Bramble Shade Forest, the border location between the Great Plains and Nanjiang. If you check the east side of the forest (the path is in the south) you'll hit a lengthy series of cut scenes, and by the end of them Yan Weihuan will join the party. You'll also receive the Crows' Cawing and Dragon Roaring manuals, and some Martial Arts Perceptions.