Ugh. This achievement returns. This will mostly be a repost, but with the following updated information.
1. The XB1 version thankfully doesn't have this achievement, only the 360 version does. So if you wanna do the XB1 and 360 cameras playing at the same time, might I suggest doing the Just Dance 2015 version of Level Headed if you haven't already gotten it.
2. I have never seen a MSFT Lounge or other store demo accounts playing this game, which makes it significantly more difficult to find idle players. Your best bet is to try leaving it on during weekend nights. There's more people playing and obviously more people likely to idle when they take their breaks.
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This will be a grind. In order to level up in the World Dance Floor, you have to get a higher score than the other people in your lobby. At the end of the song, for each person you beat, you will gain one level. So, in effect, you need to beat 2000 people to reach level 2000. The same person can be counted multiple times as long as you beat them on different songs. Obviously, getting into full lobbies is your best bet, so it's better to play at night and on the weekend when there are more people playing. Sadly, the lobbies cap at 8, and Ubisoft's servers don't seem smart enough to try to maintain full lobbies.
But, here is the lazy way I did most of it. The chair trick still somewhat works so here's the method. You'll need a chair, or some other tall piece of furniture and a jacket.
1. Hang the jacket on the chair.
2. Start the World Dance Floor, and wait till a song is in progress. When you're in the World Dance Floor menu, scroll through the users and try to get into a lobby with low level players. Generally I have found that low level players idle a lot.
3. During the song, raise your hand so that the game recognizes you. If it doesn't see the chair, pick up the chair so the game believes it's part of your body. Now put the chair down once it recognizes it as part of you.
4. Duck behind the chair and back away. If done correctly, the game will believe the chair is a person and continue judging it as such.
5. Wait till a little bit into the next song to see if the game recognizes the chair as a person again. If not, try to get it recognized again by using the above steps.
6. Now that your chair is a dancer, it will rack up some points, beating anybody who is idling in your lobby.
7. Check on it every few hours to make sure that it's still running and your chair is still dancing.
Just remember that getting a tie does not count as beating a person. Hence why the chair is needed to be recognized as a person so that what little points it gets will count as beating the people who idle and get 0 points. Plus, the chair dancer stops the XBOX from going to sleep since it thinks someone is playing. If you're not using your XBOX 360 for anything else, I recommend you always have it doing this. Also, if you want, you can do this with your XB1 at the same time to. Just connect your 360 to your XB1, snap TV, and then try to get both Kinects to recognize the chair as a dancer. If you STILL haven't gotten Level Headed on the XB1 version of 2015, try to get it along with this achievement, though there will be less people playing it as time goes on.
A good way to rack up levels is to start this during the day when you wake up, and then turn off your 360 before you go to bed to let it rest.