Batman Arkham City

Story related. If you want more info, continue reading. Otherwise, know that this will unlock through the natural progression of the story.

This mission is basically an encounter with Poison Ivy. Once you get into her hideout, you have to fight three waves of enemies. Each wave has more enemies than the previous one and the final wave also sees Poison Ivy shooting sports at you from the plants on either side of her throne. Once you get through all three waves, the mission ends.