To get this achievement, you have to play the game on the 31st of October - Halloween, but if you're like me and don't want to wait then you can use the following steps.
On your Xbox, go to Settings > Network > Network settings.
Then press 'Go offline' and go back to Settings.
Now go to Settings > System > Time and set the date to the 31st of October.
Now you need to turn your Xbox off and then back on again to make the date change. Once this is done, check that the date is Halloween and if so, load up Heart&Slash, if not, just change it so it is. When you start the game you will get the quest completion notification and you will be able to play as Heart-o-ween. I had to change my settings back to 'Go online' to get the achievement to unlock but you may not have to.