I do believe there's some sort of things to get them without so much effort.

So i try something i haven't done before when i get Abraël, i just let her at level 1 with max affection ( cause i do want to see the scenes ).
And with that Abraël lvl1 as my main girl on battle i win the 2 christmas girls on a range of 10-15 boss battles. So no more full upgrade of girls right on the bat lol.

I know rng is just rng but there's something wrong here and it can't be only luck as i played more than 30 battles/day since the event start with no drop. But with a low level all of a sudden it's a piece of cake.
Not that i'm complaining here, as i'm really super happy to get them, but i will continue my investigation on the subject cause it feels strange.

Happy hunting to all on the last day