Since no one has started one yet, I figure I will take a crack at one. Most of the achievements are straightforward, but every game should have a guide, so I will try to knock this out over the next few days. If anyone has any suggestions or techniques for the game, let me know and I will include it.
Estimated Achievement Difficulty: 5/10
Offline: 41 (925)
Online: 4 (75)
Approximate amount of time to 1000: 20+ hours
Minimum number of playthroughs needed: 1
Missable Achievements: Top Agent & Th3Glitt3rati is Online (see guide)
Do cheats disable achievements: No cheats
Does difficulty affect achievements?: Yes (OMG Indeed!)
Glitchy Achievements: Multiple achievements are reported to unlock early
Unobtainable Achievements: None
Extra Equipment Needed: Kinect Sensor, second person, Facebook account. Recommended: Kinect PlayFit and Imports from DC 1 & 2 or DLC
Welcome to Dance Central 3, the third game in what has become Harmonix's yearly Kinect series. If you have played the previous games, you are fairly familiar with this game already, though there are more multiplayer and party modes added in addition to the more than 40 new songs on the disc. You really don't need to do much in the way of preparation aside from clearing your playspace and having some water handy, but if you want to make things faster, I would highly recommend you turn off Freestyle in the options. This will give you more moves per song (except in imports from DC1) and give you a shorter grind. You can also download Kinect Playfit if you haven't already as this game will burn a lot of calories before you have the completion.
Step 1: Story Mode
The story mode consists of songs sorted into decade. In order to progress you will need to pass several hidden moves to decipher a "dance craze" and progress to the next decade. This mode isn't that long, but a few of the hidden moves can be a little tricky. First and foremost, make sure that when you start the craze, you 100% it first try (it is only the four hidden moves). If you do not manage to get one craze deciphered before you complete the story you will need to start a new save file. You get five tries though, one for each craze. Try to get as many 5 stars as possible, since you will need to 5 star every song anyway. This can be done on any difficulty.
Step 2: Multiplayer
Most of the 2 player achievements can be done with a fake person (like a coat and pants on a coat rack), but you will need another person to finish the game. Refer to the specific achievements below for more info. This won't end up taking that long, maybe an hour.
Step 3: Mop Up
There are a lot of one off achievements that you can get in this game if you haven't already, like uploading a photo to Facebook, gold starring a song, playing 20 songs with the same character and so on. Most of these are straight forward, but you can refer to the guide below if you need.
Step 4: Grinding Outfits, Nice and Flawless moves
This is going to be the majority of your time in the game. I'd estimate that it will take 300-500 songs to get everything done since the nice and flawless moves don't stack. This is the reason why I recommend you import songs, get DLC and PlayFit. With only the on-disc songs, you are probably going to need to play everything 10 times in order to get the completion, so more songs make this far less monotonous. See the specific achievements for tips on grinding.
Harmonix traded the highly skill based achievements in the previous games for more grinding ones in this game. You will still need skill to complete this game, particularly to 5 star OMG on Hard, but the game is more of a grind than anything.