Just as the description says, finish the story. After you have Unhinged Umbridge, you will go to the Ministry of Magic and go through some cut scenes. After the cut scenes you will take control of Sirius Black, and no matter what you do you are going to lose, so you might as well just sit there and take it. Once he dies, you will then take control of Dumbledore. Same scenario, you are going to lose, so if you just keep firing spells at Voldemort, he will send a mass back at you, so the more you fire, the faster you will lose. After you lose yet again, you will take control of Harry and have to use “occlumency” to defeat Voldemort. Tapping the right analog stick to move his wand back to the center. Do this, and bam, achievement unlocked.