This challenge, like a few others, it is hard for some characters, but…
Just found out ISIC’s rotating wards will kill the BoneCrusher. 
(Helix reflect option)
The BoneCrusher was chasing me, had rotating wards enabled, and as he let loose his own attack killed him.
Pleasant surprise
Also, if you use a barrel to kill an enemy, it also counts as a skill. My guess is they programmed it to be any non-buildable and non-primary/secondary attacks count as skills
Since ISIS’s charged cannon counts as skill damage, I wonder if killing the Bonecrusher (or any other skill-related objective) with it counts as a skill kill? Unfortunately, Whiskey’s ult does not.
I had problems with Ben and Whiskey.
There are tricks to killing the Ronin with melee also with some characters, default melee didn’t count.
I found it easy enough with Whiskey, though the 10 minions in 30 seconds challenge didn’t go too well.
I noticed too that on the first op that killing the pylons and snipers with Injection counted as a melee kill, strange things. I’m sure here are other oddities that we don’t know about yet.
