A Focus Attack is an attack that allows you to absorb a single hit from your enemy, and immediately counter.
A Focus Attack is done by pressing both the "B" & "Y" Buttons at the same time, if you hold them in your character will "Charge Up" before delivering the attack to your opponent. You can also do the move by quickly pressing the 2 buttons together, though the damage caused will obviously be less than holding them in.
You can do this in Trial Mode as well as in Arcade, Online etc. I unlocked up whilst playing Trial Normal with Ryu, you can spam Dan with Focus Attacks to unlock the achievement.
You DO NOT need to change your Button Config. from it's default settings to obtain this Achievement.
*EDIT*
Doing it in Training does NOT count towards the achievement.
Thanks to Dom2096 for confirming that!