LEGO Harry Potter Collection

Okay so firstly for this one you're going to need to unlock Lord Voldemort for purchase, this is done by Collecting ALL 200 Gold Bricks in the game (now im not going to put a guide here for that, thats what the Gold brick locator is for, Cheat code 84QNQN) and then walk opposite Madam Malkin's Robes to the metal gate, break it and head to Borgin & Burkes in Knockturn Alley, here you will build a gateway to a Bonus Level, once completed Voldemort will be unlocked.
Lord Voldemort costs 1,000,000studs from Madam Malkin's Robes.

Once you have him purchased start the level "The Dark Lord Returns" in Year 4 in Freeplay, make sure you select Lord Voldemort as your playable character (Harry will be auto picked).
Now make sure you are Lord Voldemort and the second character is selected on Harry (using / to change) now using Lord Voldemort switch to his Green spell (Avada Kedavra, the one with the skeleton for a logo) hold and aim at Harry. Bang, Harrys dead and you're 15G richer.

Note: This can only be unlocked by defeating Harry at the end in the Graveyard which is after walking towards the trophy after the lion+spider and yes the death eater really get in the way of this.

Here's a video

When you unlock Voldemort, MAKE SURE THAT YOU HAVE NOT 100% THE GAME YET WHEN BUYING HIM or this achievement and Quirrell Quandary will be BUGGED! I did this mistake on a previous playthrough. A little while before making this guide, I was at 75% on a new save file when getting all 200 Gold Bricks. I tried to get these two achievements after buying Voldemort and it worked!