The Hall of Water is one of the elemental dungeons in the Cairos Dungeon, and the home of the Guardian of Water. Defeating the Guardian awards players with Essences of Water. The dungeon is open every Wednesday.
The following is based on data gathered by players, as of Summoners War patch 3.2.1.[1] You can contribute by visiting SWARFARM and following their instructions!
Use skills dealing damage proportional to the enemy's MAX HP and/or Continuous Damage - typically try to use wind/water monsters that inflict Continuous damage en masse, such as Ninja (Wind) - Orochi, and look for monsters that have a high damage multiplier of the enemy's HP, such as Battle Mammoth (Light) - Marble and Amazon (Light) - Lyn.
Strike fast as the boss gets increased Attack Power for every turn that passes.
In case there is a tower that grant invincibility to the guardian, use skills that remove buffs from enemies. Otherwise, kill the tower first.
Group healer is advised (but be careful of the Disturb Recovery Crystal).
Bosses and towers are immune to inability effects.
The day that the Hall is normally open corresponds to the day in Korean, Japanese, and (archaic) Chinese. Wednesday = 水曜日 (수요일, suyoil) (すいようび, suiyōbi); 水 = Water
Hall of Water quotes[]
"Skills that cause percentile damage according to the enemy's health are very effective against Bosses with lots of HP."
"Continuous Damage effects deal 5% of the enemy's MAX HP per each turn."
"This Dungeon is open on Wednesdays. From here you can collect the Essence of Water."