This achievement is quite simple with 2 controllers.
All you have to do is go to Multiplayer, Versus, and select Deathstroke. Now while in combat just use his gun special move (Down, Forward + X) 12 times while standing next to your opponent. Once you've counted 12 go ahead and finish the match and the achievement will unlock as soon as the match is over.
Note: Be sure to avoid using his character power (B button) during this fight as it causes deathstroke to miss shots and may void this achievement.
I hope this helps.