I managed to get it, but I think I just got lucky. Didn't have a problem with it in the other ones, but in that specific combo challenge it always seemed that Kasumi either fell too fast or the throw was too slow. Maybe it's just really tight timing on that particular set-up. Damage isn't worth trying to perfect that online, lol
I've noticed Combo Challenge has a lot of combos which require frame perfect timing. Some of them being very impractical and not worth the time to learn.