Warning: Spoilers ahead!
This is the secret achievement for being beaten by Dracula, which basically means beating the third (and final) act of the game as Trevor.
Despite the achievement description, you can't just lose your fight with Dracula, you actually have to beat him fair and square. The being beaten part happens afterwards in the cutscene. ;)
The fight itself is actually pretty easy, since you have unlimited magic energy for this one. Whenever Dracula is attacking you, just switch to the same magic that he uses for the attack (Light or Shadow). That way his attacks won't hurt you that much (if at all) and sometimes you will even regain some of your health. When you're attacking, just switch to the opposite magic than him and just go crazy, just don't forget to change your magic type back, when he's preparing an attack.
It takes a little getting used to the frequent magic changing, but after a little while you will just rampage through this fight.
For the final phase of the battle, when you're falling, just dodge the red energy balls, he's throwing at you and press RT, when he's starting to glow to end the battle and finish the game.
Achievement will unlock after the credits.