Taken from the Japanese wiki:

Egor Mountains (Mistral) 20 energy, 5 battles:

Skramya (Water element), 2000xp, 1 Gem, 1 Burst Frog

She inflicts paralysis and does heavy AoE damage. Bring status immunity.

Ignia Cavern (Cordelica) 25 energy, 5 battles:

Grand Jelly (Earth element), 2000xp, 1 Gem, 1 Burst Frog

Inflicts poison and curse. Can do single target damage which ignores defence but not guard (always 2000 dmg?).

At 50% hp he casts a buff that reduces all damage he takes to 4~hp.

Standard way to kill is to POISON the boss when he casts his buff at 50% hp.

Alternative way to kill is to keep the boss at 50% hp before the buff, and then one-shot kill him.

Does INSTANT KILL ATTACK when at 40~ % hp... not sure the conditions. Post your boss experience!