Belladonnas regenerate all of their life energy each turn, along with gaining increases to their power and defense ratings when attacked. You don’t have much of a chance against them unless you can defeat one of them in a single turn. Otherwise, she will simply regain the health and come back with a vengeance. One way to defeat them easily is to defend until your characters’ WE is at 100% (use Cinnamon'sAction Trigger or Sub Tanks to heal as needed), then shift into Hyper Mode all at once and focus all of your attacks on a single Belladonna. After you defeat one, use Cooldown and go back to defending until your WE recovers again. While you're defending, the other Belladonnas’ attacks will only inflict 300 to 400 LE, so you can easily survive while waiting.
Belladonna is a type of plant. Its foliage and berries are toxic and can cause delirium and hallucinations, hence the "Riot" attack, though strangely enough the attack is only used by Rafflesian.
The name Belladona also means "beautiful woman" in Italian.
In the Help menu explaining about status ailments, a picture shows X fighting against one Belladonna in a dark location (possibly the Lagrano Ruins). Belladonna never appears alone in the game (although the others could have been already defeated), and the two locations she appears are not dark.