I dont remember fighting Kaguya :/ I wouldn't even approach a boss without a good team and i didn't even had a good team at the time. The only rewards i got was giratina wings and a halloween box, not from her. My memory is faint but i'm more confident since you said i'll get rewards from her, cause if i did even battle her i wouldn't even win. And if i did, darkrai should be available but it's not.