In the Crucible, simply kill enemy players. Once you kill 100 Warlocks, this achievement will pop. The type of death is irrelevant, can be by weapon, melee, grenade, super, etc.
As long as you get the "Kill" points to appear, you get the credit.
Note that Assists were confirmed to count. So either the "Kill" or "Assist" message needs to pop, both will count.