After defeating Penguin and his guest villain, you'll follow an Assassin back to the southeastern corner. There, you'll drop through a hole in the ground into the sewers.
In the sewers, you'll fight through some of Joker's thugs until you reach the place where the Penguin's third inference generator was. There's a door on the North side. There's more of the usual stuff there, then you'll fight a bunch of Assassins. Continue to unlock the achievement.
I don't know what it is, but I can't get the blade dodge move to work on the keyboard. Instead, I recommend double-tapping space to roll away from blade attacks.