Hannya has a decently fast attack rate, does massive damage and weakens cats to 25% from a range of 800 with no blind spot, this combined with any support in the stages that she appears in make her range and damage all the more threatening.
In Realm of Carnage and Wrath of Carnage, the only hard part about defeating Hannya is the R.Osts that come with her, the sole non-Red enemy in the stage. For R.Ost, it's advised that you use Awakened Bahamut during his backswing (the second peck) or after freezing him with the Thunderbolt Cannon, as explained in this video. Sanada Yukimura can also deal extreme damage to Hannya or R.Ost, though he will not produce as much damage as Awakened Bahamut- but he will survive longer than Bahamut.
Anti-Red Cats such as Island Cat, Crazed Whale Cat, Elemental Duelist Cat and Fried Shrimp Cat are all extremely helpful when fighting this demon, although Hannya will have to be weakened in order for Elemental Duelist at level 33 to survive a hit and you'll have to press your luck with Fried Shrimp Cat's low weaken chance. Divine Kuu or Pai-Pai are especially great options as they have anti-red abilities boosting their effective health and weaken immunity, allowing them to safely tank Hannya's hits. Tropical Kalisa is extremely useful, as she will be able to deal insane damage to Hannya when timed well.
Wore a creepy mask as a joke, transformed into a flesh-eating demon. That can happen. Breathes pure corruption born of vengeful hate. The normal Face isn't so bad anymore, eh?
Wore the cursed Hannya mask, transformed into a demon who eats the souls of those who cannot attain enlightenment. Spews grudgeful souls that cause others to lose their full power out of fear.
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A hannya is a mask used in Noh theater to represent a jealous female demon.
Amongst the first seven Advent Bosses, Hannya represents the sin of envy, as her mask is a symbol of female jealousy.
Her gender was confirmed in a Tweet made by PONOS.
Hannya is based on the king of the oni in Japanese folklore, Shuten dōji, who unwillingly became an oni after wearing a demonic mask to play pranks. It's said that Shuten dōji could survive even being reduced to a disembodied head, which is often depicted as red and massive like Hannya.
The term used for "attaining enlightenment" in her Japanese description is 成仏 jōbutsu, which specifically refers to achieving Buddhahood; attaining nirvana. This theme of Buddhism would later be carried on with Black Okame in The Pure Land, the celestial realm of a buddha or bodhisattva.
Hannya is the first Advent Boss to appear in the main Legend Stages and the first to ever be fought above 100% strength magnification.
The word Gaō in her Japanese name, Hannya Gaō (般若我王), doesn't mean anything on its own and is just a pun on kao (顔), meaning "face", though it can rarely also be a person's name. It consists of the kanji for "ego" or "self-assertion" (我) and "king" (王).
Hannya, Kappy Jr. and Kappy Kawano are the only Advent Bosses to have neither Long Distance nor Omni Strike.
While the Kappys both have powerful Surge Attacks, Hannya has no means of hitting units beyond her attack range.
Hannya is one of the few enemies with an inconsistent naming scheme in the Enemy Guide: she is listed as "Hannya(Red)".