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Story related achievement - can't be missed during playthrough.

While pursing Harley/Joker, you will have to come back to the Loading Dock of Sionis Industries and Harley will disappear up into a room covered with a Joker face shaped door. You will have to use the Remote Electrical Charge gun on the crane controls to swing the crane back and forth into the Joker face shaped door. The crane will smash through the door and then you have to pull the crane back out of the door. When you try to climb up, a short cinematic will take place and Harley will summon a bunch of goons and a one armed clown with a giant hammer. Take care of the goons first and jump away from the Hammer Clown until he is the only one left. Then you can fire the Electrial Charge Gun at him using RT + B. He will swing around and then stop. Hit him a bunch and then back off and do this again. Rinse and repeat until he is defeated and the achievement is yours.