Technical Difficulty of Ayu Songs

[hikaricore]

So a friend of mine and I were arguing over the
technical difficulty of Ayu's music versus his favorite
genre (heavy metal).
He said to give him two examples of Ayu's vocal prowess,
One of her in concert, one recording.
So I'm wondering what everyone thinks is her most technically
difficult song? And what her best concert performance was?
(I thought Kiss o' Kill on tour of secret was pretty impressive but does anyone know of a better vocal?)