Story related. If you want more info, continue reading. Otherwise, know that this will unlock through the natural progression of the story.
This is the next part of the story after you regain control of Robin. This will unlock after a huge open fight scene on the dry docks (which would make an awesome predator map, by the way). It’s like the last Catwoman fight in that you have to take down Harley four times while dealing with some of her thugs. Take out some of the thugs first (particularly the sniper) to give yourself some room to deal with Harley. Once her health bar is down to zero, take out the rest of the thugs to unlock the trophy.