To add to the above solutions, make sure the round is set to 2 and the time to 30 seconds. Then choose Ladder from the Fight menu. I choosed Sonya because she has some nice and fast moves but you can use any fighter. Make sure the difficulty is set to beginner.
Before the loading screen, you have the whole fight ladder displayed to you, showing you the different fighters you have to beat and the arenas where you will fight. If the first arena is not the Living Forest arena, wait until the fight starts then press start and go back to the main menu.
Repeat these steps until the first arena is the Living Forest one. When it is, start the fight and press the Back button rapidly while pressing any of the attack button to hit the other fighter. If Classic Smoke doesn't shows up behind the trees during the whole match (2 rounds in this case), win both rounds but don't end the match with a "-ality". At the Finish Him/Her! screen, press start and choose player select.
Rinse and repeat this until Classic Smoke shows up. It will take some retries but you will get it eventually. You can use this trick for the other 3 secret achievements where you have to find and fight the hidden fighters.