Through this friendship, Harley had her immune system strengthened by Ivy, making Harley immune to most toxins. Harley taunted Gordon, telling him that once he and Batman were dead, the Joker would marry her. Once he's down, deploy the Voice Synthesiser on the voice-activated lock to open the gate. However, Harley's hopes were soon shattered when she overheard a conversation between some guards that not only confirmed the Joker's short lifespan, but also a conspiracy to have him killed. Harley then regrouped with Frank Boles , who had taken Commissioner Gordon hostage.
What happened to Harley Quinn after Arkham Knight? : BatmanArkham
This is my "me" time. With this, she killed and replaced one of the guards who planned to kill Joker and rescued him right in the middle of the attempt on his life. Harley was sent back to Arkham without Joker who was thought dead after the fight against Deadshot. After another taunt from Harley, the guards came down to aid Deadshot whom they still thought was part of their staff, and left a rather miffed Joker behind. Do your friends call you "Harley"? Batman was sent in to save the hostages, but was captured by Harley, who locked him inside of a giant statue of the Joker inside the shrine that she had built for him. Batman eventually tracked Gordon to the entrance and found Harley humming to herself, but was unable to reach her due to an electric security gate blocking the entrance.