Simple enough, and story related so won't be hard if you're working on the game. Difficulty doesn't matter.
When you reach the top of the tower (Main Tower Outside) you will have to enter an arena to fight the Succubus. Beat her down, and the achievement is all yours.
This is the final boss for Chapter 1 (Simon Belmont) so if you have collected all of the collectibles throughout his campaign you will also unlock "Simon's Quest".