Unmissable storyline achievement, can be done on any difficulty. There's no boss fight here, just lots of enemies. Then, there's a cut scene of Batman "captur[ing] and lock[ing] up Harley Quinn." Achievement should pop immediately thereafter.
There's only one puzzle leading up to her, and it's pretty simple. Hack the panel to the right of Harley, cut the guy loose. Then hack the panel left and behind Harley, and cut that guy loose. Last, hack the door you entered through to escape. The puzzle will end as soon as the hack is complete, so don't worry about leaving time for the guards to get through.
Watch out for the prisoners with stun batons; they're easier to spot when you're not in detective mode. You can't just attack them, even if they're not prepared, they still manage to shock you. If they're unprepared, you can remove the baton with a batarang or batgrapple. You can also evade over them and attack from behind.