Why not a bonus boss... Glaice? Just... she appears, three phases, then vanishes like she never existed.
EVAN: "... what the hell was THAT all about?!"
Bonus boss, Evvie, don't stress 'bout it.
EVAN: "Did she... just call me 'Evvie'...?"
Themes in mind: "Last Battle?" (La-Mulana); "Awakening" (La-Mulana); "Boss" (Persona 2)
Where? ... Entrance E, perhaps?