Complete single-player arcade mode without losing a round.
This achievement requires you to complete arcade mode without ever losing a round. Even worse, you can't use a second controller to press start and save you, it will void the achievement. Difficulty settings do not matter but even on the easiest setting the CPU is still painfully cheap. Defeating all 12 characters in the game without losing a round will require strategy, fortitude, and a little bit of luck.
An important tip is the opponent you face first, which is random. The more opponents you defeat, the tougher the following opponents will be. Your first opponent is always the easiest to defeat. So try to start up Arcade until you get the opponent you have the most trouble with first such as Ryu or Ken, they tend to give people the most problems. Below are some the most popular methods of unlocking this:
Most people like to go with the Japanese protagonist of the series for pure simplicity. Hadouken spamming at different speeds coupled with CPU time out cheesing really helps. His Hurricane kick works very well against people like Zangief. Lure the enemy in with fireballs then hit them with a Dragon Punch for good anti-air. Use those along with some well timed jumping heavy kicks followed up with a sweep. This method will take more skill as a Ryu player than it will luck however. You'll need to know he CPU well if you want to go with Ryu. For novices, go with the Bison method instead.
Simply spam his Psycho Crusher move which is done by holding Away from your opponent for 3 seconds then press Toward them + any punch button such as . The punch button you use is important because the CPU reacts differently to the light, medium, and fierce variations. For example Ken tends to fall for the medium version more often than the fierce. Remember to spam, but spam accordingly as some of the CPU characters can find ways around it at times.
Although he's not as popular to go with as Bison some have had luck with obtaining this achievement by using Blanka. Just spam his Rolling Attack which is done by holding Away from your opponent for 3 seconds then pressing Toward them + any punch button. Blanka also has an Electric Thunder move that can be done by rapidly pressing any punch button. That as well as his crouching Fierce punch(Down + ) has good reach so use it to your advantage.
Turbo Controller(Time-Out) Method
If you happen to own a Hori-pad, Madcatz turbo controller, or a fight stick with turbo functionality then this can also work. If you set the rapid fire on some of the attack buttons while a few feet away from your opponent this will cause the CPU to sit in a crouched or stationary position. Depending on the character and stance you are using, you can usually use this as a nice Time-Out strategy to defeat opponents. Just make sure you have more health left over than them. This trick varies against certain CPU characters so check out THIS thread to see how exactly its done.
Don't give up. This achievement will take some time and patience to pull off. The CPU is incredibly cheap in this game and it will take practice to get it just right. You don't just spam these moves but you use them accordingly. A lot of trial and error will go into analyzing some of the CPU characters and finding the easiest way to take them all out. For a full guide on this achievement see THIS thread. Credit to LLC2203 for the write up.