hei there
i had exact same problem
although im not sure if it is the same case
as i was carried at my try on it
at first it didnt update it niether for me
but second try i was sure to land a hit on her before the killing
and after it went ok updated kill and hell 2 challange was unlocked
so it could be first try error or maybe it is to have a tag by hitting boss prior to getting him down exteranlly
so gl