Batman's arch nemesis runs amok in Arkham Origins. Here's the best way to tackle this mission.
Access the Gotham City Merchant's Bank
Exit the sewer from the back hall. You encounter three men with bunny masks who ask to join them at the Hat Shop in The Bowery. This opens a new side-quest for you, which we'll cover later. To get back to the open world, hack the panel to reveal the password Hat Shop with your Cryptographer Sequencer. You learn of another side quest at the Jezebel Plaza. This triggers a series of murder cases for additional XP.
Head to Merchant's Bank at The Bowery. From the rooftop, you'll spot a weak wall that can be destroyed using Explosive Gel. From there, enter the bank from the flap on the floor. Whip out your Disruptor and disable the jammer box, then hack the door with the password Open Sesame to finally enter the bank. Look for a vent you can grapple with your Batclaw and blow the floor with Explosive Gel to reach the next area. You are presented with a cool cut scene and get a new primary objective: Apprehend The Joker.
Apprehend The Joker
The Joker kills a bank employee, and Batman finds himself faced with some of his minions. There are various ways to take them down. Use the environment such as the windows, vents and the statues above. Take note that you need use your Disruptor to destroy the signal jammer on one of the guards.
The remaining thug drops his weapon – interrogate him. Joker's thug informs him to head for the Steel Mill. When the room is clear, check the body of the bank employee and scan it. You learn she was poisoned by The Joker's gas. Sadistic, indeed.
Investigate the Steel Mill
Exit the bank, sneak past or deal with the SWAT men and then proceed to the Industrial District marked on your map. The mill's gate is heavily guarded by Joker's men, so take them out easily by disabling their guns with the Disruptor. When finished, head inside the Sionis Industries door to arrive in the Loading Bay area. Several foes are present inside, but none of them are a real threat. Use standard attacks and evade as usual. Then proceed through the northern door to reach the second level of the Loading Bay.
The Sionis' Office is guarded by a machine turret. To get around, use your Batclaw and open the vent above the door. Disable the men's guns and the turret before attacking. Then access Black Mask's Computer to acquire Janus' security code. Now make your way back to the Loading Bay and hack the northeast door. The code is "Chemical." Head inside the Loading Bay Depot leading to the Warehouse.
Defeat the baddies inside the warehouse. More enemies emerge from a secret passage, including an Enforcer. This guy is tough as nails, and can only be stunned using a Super Cape Stun (B, B, B or Circle, Circle, Circle). Afterward, press the crane switch on the beam nearby – the crane's load will crash into the wall, but only crack it. Grab the load and pull it with your Batclaw to fully destroy the wall.
In the Conceal hall, scan the corpse in the elevator with your Evidence Scanner; another poisoned victim. Take the elevator down to the Hidden Facility. Defeat the guards present, then hack the western door to reveal the passcode Narcotic. Inside is the Drug Lab. Deploy stealth and take out the men quietly. When clear, enter Black Mask's office and watch the cut scene.
Scan a Sample of Copperhead's Poison & Reach the Antidote Drop Point
Copperhead poisons Batman. Turn on your Evidence Scanner and examine the perspiration on the floor. Then replay the encounter with Copperhead to spot a drop of poison on the pipeline above you. Our hero needs an antidote.
Batman moves slow and starts to show the effects of the poison. Head back to the warehouse via the elevator. Along the way, you witness some hallucinations. At the warehouse, you encounter a fight against Copperhead.
Copperhead produces clones of herself. You must attack all of them and evade their threats. Use quick jabs and counters; eventually, you should land a hit on the real Copperhead; she backs away when you hit her. Also, keep in mind your opponents will attack with their claws; use the Knife Evade to save yourself. If Copperhead grabs you, tap the button indicated to retaliate and pummel her.
Further in the battle, Copperhead will perform a rush attack. If timed correctly, you can evade this maneuver when indicated. Otherwise, jump out of the way to avoid this threat. When her health gets low, a cut scene is presented – press the button prompt to finish this assassin for good.