I couldn't finish 'em. Looks like this's gonna get complicated.
The Thunderclap Guivre is fought in a flooded room. At 80% and 40% HP, it will run to the center of the room and electrify the water. Anyone standing in the water will suffer damage over time from Electrocution. While in this stage, the Guivre will become immune to damage and bombard the party with an AoE Electrify. In addition, two waves of three Lightning Sprites will spawn and attack the party, and the Guivre will remain invulnerable until the Sprites are defeated. It is recommended to tank the Guivre and Sprites near the ramps leading into the room so that the party can get out of the water and avoid Electrocution. This is however a fairly cramped area, leaving the party vulnerable to Electrify. Beyond these phases, the Guivre's only other attack is a cone-shaped AoE called Levinshower.
Upon the Guivre's defeat, the water will erupt into an Aetherial Flow, allowing the party to teleport deeper into the dungeon.
Thunderclap is a crash of thunder.
A guivre is a mythical creature similar to a dragon. In legend they were portrayed as serpentine creatures who possessed venomous breath and prowled the countryside of Medieval France.