Massive Spoilers for NA Release; Doppel Spoilers for JP

In Kamihama, the setting of Magia Record, there are two things that set the area apart from the rest of the world.

First, there are supernatural creatures called Uwasa. Uwasa are formed by some rumors, and serve to "guard" the rumor by making it true. At least some Uwasa are extremely dangerous. They form labyrinths and have guardians similar to familiars, but unlike witches they don't drop grief seeds, implying that they were probably never human.

Second, magical girls don't become witches if they run out of energy. They can instead project the corruption in their soul gem as something called a doppel. Gameplay wise, it's pretty much just a normal (if extremely powerful) attack, but in the game's narrative, it's similar to a temporary witch. Using a doppel is, at least the first time, extremely disturbing. It takes over almost completely, and some are said to cause serious, irreparable damage to a large area (Kaede's doppel, Zola) or to attack allies (Sana's doppel, Theresia). Still better than becoming a living embodiment of human suffering.

The problem is, these two things are somehow connected. This connection hasn't been explained in the NA release yet, but the awareness of it has led a group of magical girls called the Wings opf the Magius to protect Uwasa. At least one of their leaders is a dangerous sociopath, but most are apparently normal magical girls who just really don't want to become witches or see that happen to their friends/loved ones. It seems significant that, of the protagonists, only Yachiyo is actually aware that becoming a witch is something that could happen, and she doesn't seem eager to let anyone else know.

Anyway, all that to say that the green thing is the Uwasa of the Anonymous AI. She's an odd example of an apparently conscious Uwasa, who was promoted by a website run by the Wings of the Magius. Initially she behaved like other Uwasa, but after meeting a girl named Sana Futaba, she has become significantly more empathetic. Because she believes that Sana will be happier if she is released from her digital prison (where Sana has taken to calling the Uwasa "Ai-chan") but realizes that she will revert to her role guarding a rumor after releasing Sana, Ai asks Iroha and the others to destroy her.

This version of Ai is from an extra episode in a side game called Mitama's Special Training. She has an attack that essentially kills any of the player's units every 3 turns, and extremely high health. The OP is attempting to use multiple doppels to kill her before she can start knocking off units one by one, to accomplish the goal of beating the episode without using a continue.