I don't usually write solutions for story related achievements, but the Crow Witch is a bit of a tricky fight, and it took me awhile to even figure out how to hurt her. The achievement is technically for completing the Chapter, but I'll just cover the boss fight here, as the rest is self-explanatory.
After the cutscene, the Crow Witch arrives and she'll start hurling large eggs at you which will break open and spawn opponents for you to contend with. The trick here is to grab the eggs and hurl them back at her - as many of them as you can. Hitting her with the eggs will stun her, so run up and grapple onto her. She'll fly up in the air, so wail away on her, and then execute the QTE when she dives back down.
She'll now fall to the ground, so run up and pound on her face until she falls off and flies up in the air again. Run around and kill any of the small enemies that you missed throwing back at her earlier, and wait for her to start hurling eggs at you again. Repeat the process until she falls down onto the ledge again. Run up and grapple her again, and pull her freakin' head off.
Watch the cool cutscene, and enjoy your shiny new achievement.