You have to defeat a total of 10 enemies in a row with EACH of the 30 required characters. Counting the DLC characters you have a total of 36 characters to choose from (you don´t have to do them all).
I did all of the characters because I wanted all of the icons but that´s up to you. The most effective way to get though time attack with each character is a jumping strong kick followed by a crouching strong kick. The enemy will fall for it most of the time and if you´ve got the timing down you can actually lock them in the corner.
The amount of health you gain after a match depends on how you have finished your opponent. While normal and special moves don´t give you back that much you´ll notice that Super Special Moves or even a Neo Max will make your life a lot easier.
I made a video showing the "technique" by a request of a friend because people kept complaining that it doesn´t work for them. All you have to do is playing with patience. Sometimes the opponent will go all out on you. Just let them rush and block, analyze the situation and keep going.
IMPORTANT: In the video I´m only using jumps and standing/crouching strong kicks (which a few exceptions which I did on purpose). It´s only to help people who have never played any fighting games or just want to have it really simple. You can cut down a lot time if you use super specials, evade rolls and such. Please keep that in mind.
The timing and execution of the low strong kicks vary from character to character so I will list the ones I had the most touble with (but the majority will work as shown in the video).
Chin, Clark, Mature (I used her 214LP instead), Ralf (generally most of the slow heavy hitters but they have a lot of health on the other hand).
It´s been a while since I got the achievement so please keep that in mind. If you have any questions, feel free to drop me a message. Many thanks for reading and good luck!