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 skelly's and if she gets low on HP run away a bit and she will reset.
Attack using only that skill I think it's 1,000 kills but felt like more.