I currently have the quest to kill craged adult yetis which is fine. If i kill one of them it counts up. The problem is, when using the rage mode to kill them. These kills does not count. There is no text that says you have to kill very slowly with your sword. And and least in older versions killing enemies with rage mode also counted, so I would expect the same behaviour here
#charged adult yeti rage kill
ancient sluice
brittle birch
You don't have enough rage mode damage so you are not killing them
Check your stats you need over 250
jaunty current
Wow the description of rage mode sure sounds like you’re supposed to kill everything. I didn’t realize there was a damage value associated with it. That’s a bit misleading.
proud quiver
The time between unlocking Rage Mode and this event occurring is several months apart 
The "Rage Mode Damage" is in the stats menu