First, you will need to find the Spider Venom relic.
Then, head to Barron Hypno's Castle and work your way through the castle to stage 2. Jump across a few of the eyeball blocks and you'll come across a half spider half human hybrid. This is a Bonelegs enemy.
Transform into the spider and hit B. You'll find that they defend quite well. You could be all fancy and jump left to right but if you have enough health vials/food just run into the Bonelegs and hit B. When they rotate to face you face away. You are basically aiming at the spider abdomen.
Few good shots and down they go and you get the Achievement.