Must be completed _cretera_ instead achievement and using proper call instead hacks like you attempt do...
Player have
Player::UpdateAchievementCriteria
But in most case like kill boss with additional reqirement expected some another way... and this call will propertly ignored.
Achievment criteria for kill special reqs check additinal requirements by achievment_criteria_data. In many kill boss cases not exist achievement creatria data types for descrive its.
Possible need implement achievement creatria data type *_SCRIPT that will suggest achievement system ask instance data for additinal reqs for criteria...
And in instance data virtual function you will can describe check like some boss killed at moment, anotyher boss alive, none death while phase, some objects not destroyed...