This requires the Sharpshooter class.
This skill is Tier 1 in adventure tree skill name Scout's Bones (Active Skill)
I suggest going to act 2 Corpsefire Crypt in Polluted Plateau and find the Boss: Sylvia Rouser, Necromantic Summoner she will keep summoning more skellys and if she gets low on HP ran away a bit and she will reset.
Attack using only that skill i think it's 1,000 kills but felt like more. See short video below for ideas.
Sorry, this Game Clip has been removed