Is this your first time doing the quest? If so, it is a different Manon cry that drops. The Manon cry you would want (if it is the first time) is acquired by killing Manon without other players inside the map, as you have to finish it off ON YOUR OWN.
The short answer, if you had someone helping you kill that manon you need to have them leave the map before he dies, and you kill it.
