Batman Arkham Asylum - Party Pooper - KO all the henchmen celebrating your arrival at the party

Missable achievement - unlocked at the end of the game.
There will be some enemies outside the building who won't attack you, so place the explosives (must be upgraded to make place three at once).
After you get through this.
Go inside building corridor (those you meet inside, same as those outside also not attack you as first, so beat this guys cause it's easy and achievement will pop).
If you play well inside, you will get the 20 combo achievement.