As of right now, no one has this achievement or the following four achievements which make up this achievement:
&
Although no one can verify this, it is assumed that in order to unlock all accessories you'll have to beat all of the challenges (as defeating each challenge nets you an accessory), in addition to reaching an unknown number of "Followers" (probably at least 50,000 since there is a separate achievement for that).
The problem with the challenges themselves is that they are completely random. When you are lucky enough to get a challenge you'll see a graphic of one of the BEP members alongside the details of the challenge. The BEP member depicted hopefully indicates which member is proposing the challenge (i.e., for the "tight with" achievements). You might as well select it every time as there is no reason not to.
Anyway, what I thought I would do here is compile a list of all of the known challenges. If anyone has been faced with a challenge that isn't in the list below, please indicate this in the comments and I'll try to make an effort to keep this list updated.
So, here is the "hopefully temporarily" incomplete list of challenges:
1 Clear with rank B or higher - Strike the final pose.
2 Clear with rank A or higher.
3 Clear with rank A or higher.
4 Clear with rank A or higher, Strike the final pose.
5 ?
6 ?
7 Play in co-op mode - Clear with 96 or more Followers.
8 Clear with 112 or more Followers.
9 Play in co op mode, Clear with 128 or more Followers.
10 Clear with 160 or more Followers.
11 Clear with 192 or more Followers.
12 Play in co op mode, Clear with 224 or more Followers.
13 Clear with 256 or more Followers.
14 Play in co op mode, Clear with 320 or more Followers.
15 Play in co op mode, Clear with 344 or more Followers.
16 Play in co op mode, Clear with 408 or more Followers.
17 Clear with 464 or more Followers.
18 Clear with 584 or more Followers.
19 Clear with rank A or higher - Strike the final pose.
20 Clear with 60 or more "good", "great" or "incredible" dance ratings.
21 Clear with 50 or more "great" or "incredible" dance ratings.
22 Clear with rank S.
23 Earn 40 or more "incredible" dance ratings.
24 Clear with rank S - Strike the final pose.
25 Earn 30 or more "incredible" dance ratings.
26 Earn 40 or more "incredible" dance ratings.
27 Clear with 45 or more "great" or "incredible" dance ratings.
28 Clear with 60 or more "great" or "incredible" dance ratings.
29 Earn 20 or more "incredible" dance ratings.
30 Earn 30 or more "incredible" dance ratings.
31 Clear with 10 or more smooth transitions.
32 Clear with 15 or more smooth transitions.
33 Clear with 20 or more smooth transitions
34 Clear with 10 or more smooth transitions.
35 Clear with 15 or more smooth transitions.
36 Clear with 20 or more smooth transitions.
37 Play in co-op mode - Hit 7 "in sync" ratings.
38 Play in co op mode, Clear with 10 or more sync bonuses.
A few things to note about the challenges...
If you want to see the list of the challenges you have attempted, you can find them (whether you passed or failed them) under "Career" ===> "My Dancer" ===> "Challenges". Clearly, a "star" on the left-hand side of the challenge's number indicates that you have already passed the challenge.
Once you have beaten a particular challenge, it will not be proposed to you again. If you fail it though, there is a good chance that it'll be the next challenge you see - until you pass it.
If you are nearing the end of the song and you think that you will fail the challenge you can pause the game (with the controller or the "Kinect" way) and restart the song without deactivating the challenge. The challenge is only deactivated by finishing the song or exiting the song outright.
The higher the cumulative difficulty of a particular song (i.e., both the song's difficulty rating, as well as the general difficulty you are playing on) play a role in determining the number of Followers you can and need to get.
You net way more opportunities to get "smooth transitions" if you are dancing the routine for the entire song (i.e., not Step 1, 2 or 3).
Many of the challenges appear to be "difficulty" specific. Meaning that if you are only dancing on "Human" difficulty, you won't get the really hard challenges - they'd be impossible anyway. (Note: Human has a 1X multiplier, Machine has a 2X multiplier, and Alien has a 4X multiplier).
If you are seriously trying to focus on these achievements, I suggest picking a harder song, that you are good at and doing it on "Alien" over and over again. Eventually, you will get a challenge proposed to you and when you do, you will be better prepared to beat the challenge. Again, if you are nearing the end of the song and you are unsure if you'll beat it, restart the song.
Finally, several challenges are co-op (which would explain why several are still unknown as they'll only get proposed to you if you are playing co-op). The problem with co-op is that you can't just have a chair or your 5-year old son be your partner and hope to have any success. In order to gain many followers in co-op mode, you will need to have a partner who can dance well with you. The score you attain in co-op mode is based on the average of both dancers grades. For example, if you get an 'A', but your partner only gets a "C", then expect to have a "B" as your overall grade.
Note: I'd personally like to thank Michael Jackson...errr...Dr Tinus NL, for his help with the solution. If he can't get this done, then noone can! hehehe
Note 2: Per hoLLowPoint75: "I've been on Alien for quite a while now, and I'm convinced that difficulty only has an impact on the followers that can be gained, not the challenges that are issued. I've received challenges for 112 followers and 128 followers in co-op whilst dancing on Alien."