One Step From Eden

You can set your deck focus to Miseri for the best shot at finding Devour; it's a higher rarity card so some luck will help. Once you have it you find a hostages either in a regular node or in a distress call node where they're on the enemy side. Devour only does 80 damage so you'll need to weaken the hostage just a hair first, then cast Devour to kill them for the achievement. Devour is consumed on use so you'll only get one chance per battle.