The End

[Big D]

the end does not work on ultimecia's final form,

It doesn't work in quite the same way, but it does have an effect. You know how Ultimecia starts talking when you've nearly beaten her final form? After that, she says another line each time you attack. Well, you can skip the first part of that fight and go straight to the talking by using The End. I tried this myself :-

Ultimecia: "I am Ultimecia... time shall compress... all existence denied".

Selphie: *The End*

Ultimecia: "Reflect on your... childhood..."

and so on.