Harry Potter for Kinect

This achievement requires you to complete all of the multiplayer mini-games. These are local multiplayer not Xbox live so this achievement is offline only. As stated in the other two solutions they can be completed solo and for some seem to be slightly glitchy when they unlock. According to the other solutions they also don't need to be won, just played. I did mine after completing the story line and unlocking what I believe are the only 3 mini games. I also had to complete all 3 before the achievement unlocked. These are the games and what you need to do to complete them.

Competitive Quidditch Seeker: Both parties fly along the route of the Golden Snitch, once you have gotten close enough to the Snitch the grabbing section is activated and you must hover your hand over the snitch until the meter fills. After you grab the Snitch the winner is announced and the game is complete.

Competitive Dueling: This is a duel between the two local players. Simply spam Incendio at the inactive player until their life is depleted. I had to step into the second players area as they started with Protego active and that prevented me from damaging them. Once I did that they dropped the shield and I continued until they were defeated.

Co-operative Dueling: This is a duel where the two local players take on the two enemies from the Luchino Caffe Duel. I ignored the second player completely and just defeated the two enemies myself. I do not know if both local players need to survive but I had no trouble defeating the two enemies before my second player was defeated. Once both enemies drop the mini game is complete.

Once you have completed the 3 games the achievement will unlock. Speculation suggests the achievement may unlock once you have completed the Multiplayer games you have unlocked however I do not know if that is actually the case. If anyone can confirm I will update accordingly.

As said above you can do this alone and don't need a second player. But when I got this only one event was unlocked, I played that one event and somehow still got the achievement but I'm not sure it works all the time, it may just be a glitch of some kind

You can do this yourself, you don't need a second player. In fact you don't even have to win. Just play each game and the achievement should unlock.