This is one of the basic achievements which should pop for most noobs in the first fistfight against Joker's henchmen just after he breaks free.
Before we start, let's recap.
Left Mouse Click - Strike (punch a dude)
Right Mouse Click - Counter (when little squiggly lines appear above a henchmen's head, jam this button to stop him from smashing you)
Double tap Space + Direction - Evade (do a little dodge from side to side. This will not add to your combo)
Middle Mouse Click - Stun (flick your cape. This will not add to your combo)
So yeah, just strike and counter the first three dudes in that little room you get trapped in after Joker escapes. Look, the TA Ratio is 100 so really I don't think anyone should have any problem with this achievement