This is the ultimate attack move, if you played other fighters it is similar to "super" attacks. You need both of your blue special bars filled to be able to execute it. It is always towards the enemy and the
S button. So if you are on the default (left) side of the screen, then it is
. As with other super moves in fighting games in general it has to connect the first hit to start the actual move (the background goes black and your character charges up a devastating hit, then the camera closes in the point of impact). This means this move can be blocked or countered (mid)! To reduce the risk of getting countered and losing the meter it is advisable to prepare for this move with
SSSS aka the Fatal Rush attack (
). After the first hit landed from the Fatal Rush (which can be still countered or blocked but doesn't use up meter so you can risk it freely) the rest can't be stopped EXCEPT with a Break Hold (away from enemy and
S) so if the enemy doesn't have a meter or doesn't know how to do it then he is toast. After landing the 4
S press it again but with holding forward against the enemy. Fatal Rush get auto combo'd in Break Blow. Great way to end a match quickly if the enemy is low on health.
I got several of these
S button achievements after a single ranked match (3 out of 5) so I believe the text is misleading and it is round based. But since you get it at the end of the match (not right away) you may not get anything if you lose in the end.
As the achievement says you have to do this against AI or online opponents.