Currently there are 350 Trials available (including 98er Iori and NESTs Kyo as DLC) which means that you have to do roughly 6 Challenges with each character in order to get the achievement. With the upcoming DLC character Mr. Karate later this year, the trials are going to increase to a maximum of 360.
I think it´s really important to state that DLC trials DO COUNT TOWARDS the value you need to reach (I´ve tested it and it was confirmed by a friend of mine as well yesterday).
So what´s this solution all about you may ask? I´ll write down the trials I´ve finished and go a bit indepht on some characters since there are a lot of shortcuts for most trials. I´ve also tried to create a playstyle section for specific characters so that you can go all nuts when you got a specific mechanic down (charge characters for example). All examples are expecting your character standing on the 1P/left side and facing right.
To give you a small idea of what to expect from the characters, I rated them with 1 to 5 stars with 5 being very hard. Please beware that this rating is purely based on my own experience and some of you may find a specific character easier/harder than I do. Best example for me would be Elisabeth who I would have given a 5 doing my start of the game. But then when I got her Drive Cancel down, she´s a smooth 3. You get the idea.
Character - challenges finished - difficulthy
Raiden 6/10 1/5
Ralf 7/10 2/5
Clark 6/10 2/5
Leona 4/10 5/5
Ash 4/10 5/5
Charge moves are often performed by holding a specific direction for about a second and then input the opposite direction together with an attack button (best example here would be Ralf´s 6HP.
The normal charging method won´t help you during most of your combos since there´s not enough time. That´s the point where the buffer system and charge partitioning comes into play. Let´s take Ralf´s Challenge 3 as an example:
2LK HP 236HP 2HP 6HP 236236LP/HP (first link)
After the 236HP 2HP part, you don´t have enough time to input and hold 4, then release 6+HP. The key here is a shortcut which is involved with every charge character and essential for him/her to compete. You have to input the combo like 2LK HP 236HP and then hold down , input HP as 1HP and immediatelly press 6HP as soon as you hit. The 1 counts as both down and back presses during your combo and it´s the only possibility to connect this combo. This may seem odd at first but you´ll get into it sooner than you think!
Raiden´s trials can be a bit tricky so I give you a few tips on them:
Trial 3: Input the 2 half circles normally and press LP each time you reach 2 or 3 and then input your super as usual. It should then look like 63LP214 63LB214 LP. This one is a bit unusal at first but you´ll appreciate it since there´re a few more trials like these (Saiki 5 for example) and it works like a charm once you´re familiar with it.
Trial 5/6: Raiden was nerfed in the console version and the required lvl 3 Dropkick shown in the demonstration will only come out if you hold LK and/or HK for 24! seconds and then release it (kinda like Hilde on Soul Calibur IV). Yep, 24 seconds. I recommend you to map both LK and HK to your shoulder buttons and set up a clock or count the seconds manually. After you got that down, trial 6 is a breeze.
Trial 5 is a bit tricky. The timing for the LP charge and then EX-canceling another charge is really strice so can map a kick on your shoulder button, perform the combo as normal. After the first LP charge, release your lvl 3 Dropkick BUT keep charging  and then release your 6LPHP EX Charge. Training and patience will let you reach this one pretty fast. Doing trial 6 before will make it easier.
Mature 7/10 1/5
Kensou 6/10 2/5
Duo Lon 7/10 4/5
(DLC) Iori 4/10 3/5
This term comes form Fei Long of Street Fighter. Rekka combos are performed by chaining together forward/backward quarter circles + punches/kicks up to 3 times (with the addition of EX moves, the number of available moves varies). You can usually delay each hit manually (link 2).
There´s not much to say here except what you should do on every trial. Don´t mash and perform each move as clean as possible. Duo Lon will be the hardest of these since he requires both precession and speed. If you´re completty unfamiliar with Rekka characters, Duo Lon will seem like a 6/5 but he´s not that hard. Practise and patience are the key again.
Art of Fighting characters
Ryo 6/10 2/5
Robert 4/10 3/5
Takuma 4/10 4/5
Each of these fighters has a unique input which is 646 + punch/kick. You´re most likely going to cancel these motions after uppercuts and such. The key here is to inut the uppercut as follows 6236LP/HP. This way, the last input from the uppercut will be registered as the first input of the 646 motion. Should look like 6236LP 46LP if done right.
Keep in mind that both Robert and Takuma also have charge elements involved during their combos which is why I rated them harder than Ryo.
Vice 10/10 1/5
Maxima 10/10 1/5
Goro 8/10 1/5
Saiki 8/10 2/5
Billy 8/10 2/5
Self-explanatory. Maxima doesn´t have any hard links and his combos are short. He only has one short and easy Hyperdrive (HD) combo (trial 10). Vice on the other hand has long combos and more HD cancels BUT her timing isn´t strict and the pattern of her combos stays the same until the hand. I highly recommend her, she´s really fun to play and you can also use 41236LKHK which makes her earlier Trials much easier. Examples:
Trial 1: 2LK 2LK 41236LKHK 632146LP/HP
Trial 3: 2LK 2LK 2LK 41236LKHK 6321463214LKHK
The later combos involving a forward dash HK (after 214HP) into 41236LKHK can be simplified if you do it at the corner and simply delay the HK a bit. Be aware that you have to REVERSE the input of her Neomax in Trial 10 if you have started the combo from the left side (link 3)!
Saiki has a unique style since he can hold down one of the 4 attack buttons after his 214HK in order to warp himself to different locations (note: he only does that if he hits the opponent). Seeming odd at first, you´ll get used to it once you´ve finished some of his challenges. If you have problems with trial 5, use the same strategy as written earlier with Raiden or cancel your second 2LK into Hyperdrive mode and then do a HP (altough I don´t recommend this method).
Elisabeth 6/10 3/5
Shen 7/10 2/5
Kyo 4/10 5/5
Benimaru 6/10 3/5
Iori 6/10 3/5
Terry 6/10 2/5
Andy 5/10 3/5
Joe 5/10 2/5
Athena 6/10 1/5
Chin 5/10 2/5
Kim 7/10 1/5
Hwa 4/10 3/5
Mai 7/10 1/5
King 7/10 1/5
Yuri 5/10 2/5
K´ 5/10 5/5
Kula 5/10 3/5
Practice is the key. If you just started the game, some characters will seem impossible to you. Don´t let that pull you down! A trial is getting on your nerves and you don´t get it down? Skip it and try another one, you have to be very patient sometimes. Besides that, try to execute your inputs as controlled and clear as possible. Mashing won´t get you anywhere in KOF.
If you happen to get to a combo where you can´t connect 2 attacks like LP into HP or j.HP into HK, it´s most likely a link. A link requires you to connect 2 attacks with a specific timing. Elisabeth´s j.HK HP is a good example or Shen´s LP HP in Trial 5.
Instead of rapidly pressing the buttons, execute the first one and wait for a slight amount of time (let´s a say a good second) and then input the second attack. Vary your timing each time until you get a clean hit. Watching the demonstrations carefully will give you a good idea more than just once.
If you have any more questions in general or character-related, you can drop me a message or visit http://orochinagi.com/
for great and usefull advice. Many thanks for your time and good luck!