How to defeat Leviathan

I remember the day Levia joined to my party. It was such a pleasure to finish the notorious fishing*, catch and defeat every four guardians of the spots. The last thing to have Levia join is to defeat Leviathan. It is almost impossible to defeat the monster of the legendary name without one or more of 5-star Dream characters unless the version becomes around 2.0.

Actions of Leviathan

Turn	Name	Effect	Area
1	Tidal Canon	Water Magic	all
2	Dragon Cascade	Water Physical + sleep	all
3	Erase Blast	-30% of current HP	all
4	Storing power	INT buff	self

Tactics in General

The key points are (i) to mitigate damages at 1st and 2nd turns and (ii) to deliver damages at 3rd and 4th turns with Earth type attacks. As to (i), fundamental is +50% Water/all-elemental resistance buff. In addition to the resistance buff, either of the following (de)buffs should be used:

• Azami's or Isuka's INT debuff for 1st turn
• Lokido/Shion/Isuka's PWR debuff for 2nd turn
• another +50% resistance buff (75% in total) for 1st and 2nd turns

Either Mariel, Myrus or Erina + Chiyo may be required for the party to effectively mitigate the damages in around ver. 1.5 or earlier. Except at the AF turn, resistance buff should be always cast at turn 3 (or 4).

In case of Erina + Chiyo, keep them in front using Life Steel and Earth Wall at turn 3, and then use Reset and switch Chiyo with Azami or Isuka at turn 4. Keep in mind Erina's SPD should be 122.3% or more of Chiyo's so as to get Erina act faster than Chiyo. For example, in case of Chiyo's SPD is 114, Erina's should be at least 140. This warrants the erase of INT buff of the enemy and cast of INT (+ PWR) debuff at turn 4.

Example

Below is a very nice clip where the player defeats Leviathan only with four characters based on his research of the enemy.

Tactics in the Clip

Choose characters with Water resistance/Earth type attack in their ability
Myrus, Chiyo, Riica and Deirdre have all Water resistance in their ability. Myrus has +30% and the rest have +20%.

Use +50% Water resistance buff twice
To mitigate damages by Tidal Wave and Dragon Cascade, Earth Wall is always in effect double by Chiyo and Myrus: 80-82.5% damage reduction in total.
Mariel instead of Myrus is more stable since her SPD promises her first move. In this case Aurora Force of Mariel is used and the damage reduction rate for Mariel is 75%.
Note: see also (de)buff accumulation.

*Several adjustments to the fishing mini game was made to ease the completion of fishing because of so many complaints here and there, and it becomes way easy to accomplish it.