Hi I used this method to get it:
Already in settings:
Cpu Lv 1 - Play time Fast - Single round 1 - Defense Level 1p 8 - defense level 2p 1 - Power gauge Max - 2p normal
I got it in arcade mode with Ralf using LB.
( LB is a series of a few shots that is triggered (for ralf) depending on the proximity of the opponent. Ralf will punch the opponent if he is at a certain distance and if the opponent is close enough, he will continue with a small powerful combo. to better understand go into training mode take Ralf do LB near the opponent and do LB stick to him you will see the difference
)
be careful some opponents are harder to beat advance cautiously parry and after LB.
( EDIT : I add Ralf's SDM which will help you for this achievement and others )
the name of this movement is Galactica phantom
you must make 2 quarter circle starting from the bottom to the right +
.
The blow kills all the enemies in one blow
Only in the last fight does he take away 70-75%.
it is also quite easy to place.
Ralf puts himself in a position for a few seconds then runs to the opponent (over a short distance) and gives him a punch.
The only time you miss is if your opponent dodges, or catches you.
or if you are too far away but generally it's good
If you are going to lose (don't panic). Turn on your second controller and press start before losing.
Kill the 3 characters of your second controller and you will resume the fight from the start without compromising the appearance of the achievement.
( During the last fight I used my second controller 3-4 times so it works. )