The event is rather unusual and combines traits of both Exploration and King Mog events. Players navigate through a small, yet labyrinthine dungeon where their objective is to collect Lanterns which serve as event currency. Lanterns can be acquired through harvest points or defeating enemies. The higher the difficulty the higher the rewards.
However the method that yields the most Lanterns by farm is the Mermaid, which serves as the event's Boss. The Mermaid can only be found in a specific spot in a secluded part of the map. The entrance is identified by a Fish Statue. Battling the Mermaid is entirely optional but needed if one aims to acquire large sums of Lanterns. The player can leave at any time if they find the exit, identified by a large brown bamboo shoot.
What makes the dungeon particularly tricky is its 3 minute duration. If the player runs out of time, they earn an instant Game Over. They still keep any rewards acquired like those from Treasure Chests, Lanterns, etc. However if time runs out while fighting the Mermaid they will not earn her drops.
The Lanterns are then used to purchase goods from King Mog. Differently from usual events hosted by him, the stock of items is significantly more limited.
At ELT difficulty, the Mermaid has as much as 3,000,000 HP. Her ATK and MAG are decently high but her DEF and SPR are modestly low.
She attacks with Waterga for Water magic damage to all units. Blow Away for Wind magic damage to one unit and remove one unit from battle. Mermaid's Curse for instant KO to the unit with the highest ATK and lastly she damage and put all units to sleep.
She resists Water (100%) and Dark (50%) but is weak to Lightning and Light (-50% both). She is susceptible to Blind and Disease and vulnerable to all Stat Breaks.
The Mermaid is not a particularly difficult opponent to defeat, but the player will need firepower to be able to defeat her on time, while being able to survive her attacks.
A Tank is useful to deal with Blow Away. Since she doesn't use physical attacks, losing the Tank is not a real loss.
The Event Banner's featured units are useful against the Mermaid. Luka can provide Water resistance, HP Heals and Status Ailment removal (she must be equipped to be immune to them however). Mercedes' has Aquan Killer and strong ATK buffs (even more so if her abilities are awakened). Exdeath can use Thundaga and Wicked Light which hurt the Mermaid on her weaknesses. Lastly Orlandeau is a superior damage unit who can capitalize on greater Light damage outputs (he must be equipped with a Light weapon however and he is a rare unit).
If the player doesn't have access to any of them, there are other alternatives. A fully leveled Fina with her exclusive equipments can also heal status ailments, HP and is immune to Sleep (she provides no Water resistance). Stat Breakers like Warrior of Light, Ling, 9S or Timothy are all helpful against the Mermaid.
Green Mages like Cerius, Marie or Minfilia can provide protection against Water and Wind.
For damage, the player has ample room. The usual high level chainers are useful. Mages with Lightning damage are also good. If the player has Aquan Killer it will help boost the damage. They should focus on Lightning and Light damage. Bran is excellent as it attacks both of the Mermaid's weaknesses at once with his strongest move. Other elements can be used as well, though they should be employed with some Imperil of sorts.