[Senna]

Two in one bug.

It seems the Feathered Serpent. Whenever Mercalan Mist is used in the battle and it summons the rain, it becomes indestructible as well as take Mist's properties. It also obtains the healing from the mist and, due to its mechanic to not die without a suicide attack, it survives and heals beyond 1 HP threshold, causing it to be immortal. It will not die from its own attack and will always freeze its HP at 1, only to heal the next turn and repeat the cycle. 

The healing from the mist persist even after the it dispels though this healing effects every single mob in the battle as shown below. This occurs happens whenever the Mist is acting in the rain the boss summons.

---

Apparently, it can do this without the mist but it's inconsistent.

[Rendar]

This is a bug with Mercalan Mist itself.

Whenever the effect ends, EVERYONE on the field gets HP regen. Not just enemies, but every unit.

[Neus]

Seems that field effects weren't being cleared after going into effect. This has been corrected.