This achievement is for killing all of the guys inside of the hallway of the party building.
First of all, I disposed of the four guys outside, they should be no problem if you use explosive gel to knock them down for some easy KO's.
Once inside it is time to prepare. Lay down 3 explosive gel, evenly spaced in the middle of the room (I had the auto-explosive gel upgrade but this will still work without) then, I stood at the end of 1 line of guys and used the line launcher to launch to the other end of the line of guys.
Batman will launch on the line, kicking in the faces of everyone in that lines, this will cause everyone to go crazy, at which time you should blow up your explosive gel, knocking pretty much everyone to the floor. Kill as many guys as possible whilst they are on the floor, then, use your attacks, countering and batarang skills to skillfuly kill the rest of the guys.