Was that place where you got the other 30? If so, not sure why it isn't working now. If not - what I remember about it was if the Perfect Fifth shot kills the enemy, it doesn't count, if the explosion kills an enemy, it doesn't count. Only kills from the subsequent burn damage counted. Those shadow thralls are getting killed by the shots directly or caught up in the explosions. It has to be an enemy that can survive the Perfect Fifth hit/explosion and be left with enough health that the burn damage finishes them off. The Legionnaires on the Leviathan were perfect for it, not sure where the optimal place for it is now.