The Penguin is trying to smuggle weapons caches into Gotham City from Bludhaven. You need to stop him from arming the rioters as part of the Gunrunner Most Wanted mission in Batman: Arkham Knight.
You will automatically unlock Gunrunner during the main storyline. This will be right after the Arkham Knight abducts the Oracle. Alfred will inform you that Nightwing has some important intel he wants to share. Meet him on a rooftop overlooking the Ranelagh Ferry Tunnel to learn about the first cache location.
There are a total of five weapons caches that must be destroyed before you can apprehend the Penguin. Each cache requires you to track down his refrigeration trucks and follow them back to their hideouts where the weapons are stored.
Weapons Cache #1 – Harold’s Repair (Miagani Island)
Unlocked in Chapter 3, Nightwing will hand you the Disruptor which is one of the best gadgets in Batman: Arkham Knight. You use it to disable the guns of the thugs below. You can also disable weapon crates to stop enemies from arming themselves.
Take care of all the enemies and use the Disruptor to place a tracker on the truck. Knock on the truck to spook the thugs inside and then follow them. You can not trail them in your Batmobile. You can only follow them from the air. So use your Grapple Boost to launch yourself into the air and keep the truck in your view, or at least in your range. Make sure that they do not see you.
Once they believe they have lost you, they will head to their base. Follow them to their hideout, which will be Harold’s Repair on Miagani Island. Head inside the building from the rooftop. Use the Disruptor to sabotage the weapons held by enemies, and use the Remote Hacking Device to blind the sentry gun.
Once you have dealt with the enemies, slide down the hatch and interrogate Penguin. He will escape after spilling the beans about his caches. You and Nightwing will then have to clean up the remaining enemies.
You can switch between Batman and Nightwing to access their separate moves and gadgets. You will find Nightwing a bit more agile. Do not ignore the chairs and pots on the ground. Throw them at the thugs to momentarily daze them.
Once you have cleared the area, use your Explosive Gel on the weapon crates in the vault to clear the first Gunrunner location in Batman: Arkham Knight.
Weapons Cache #2 – Sionis Industries (Bleake Island)
This Gunrunner cache location becomes available at the start of the fifth chapter after you exit the Stagg Airship in Batman: Arkham Knight. Nightwing will contact you once you activate Gunrunner from the mission select screen.
The truck is parked near the Mercy Bridge on the north end of Miagani Island. Similar to before, place a tracker on the truck and follow it back to its hideout. They will enter Bleake Island and eventually park the truck around the Sionis Industries near the Falcone Shipping building.
Call the Batmobile to your location and enter the building on foot. The entrance is guarded by two sentry guns that will drop you in seconds if you try to head in straight.
Instead, use the Remote Hacking Device to raise the shutter. Now select the Batmobile Remote from your gadgets and use the 60mm cannon to take out the sentry guns. Head inside and grapple up. Use the switch on the wall to open the shutters on the roof. Head back outside and grapple to the roof to surprise the thugs below.
Once again you can work with Nightwing to take down all the enemies and then use the Explosive Gel on the weapon crates in the vault to destroy the cache to progress the quest to 40 percent.
Weapons Cache #3 – The Grinning Fishmonger (Founders’ Island)
The third truck can be found parked behind the Panessa Movie Studios in the Cauldron. This will only become available after you have dropped Poison Ivy at the Botanical Gardens in Chapter 5. Put a tracker on the truck and follow it to the northwest edge of Founders’ Island.
Head over to the Grinning Fishmonger entrance and grapple up to perform a Silent Takedown on the sniper above. Then head inside to slide down the vent to enter the building. Exit the grate on the other side of the weak wall behind the enemies to take some of them down by using the Explosive Gel on the wall.
Now use the Batclaw to remove the vent cover on the wall and grapple inside. Now from here on follow the same drill as the last two caches and destroy the weapons cache with Explosive Gel.
Weapons Cache #4 – Chinatown Sewers (Bleake Island)
Once you have dissipated the fear toxin, contact Nightwing and use the same strategy as above to follow the next van east of Perdition Bridge. You will need to take the Batmobile underground to Gotham’s sewers in Chinatown. Once you reach the marker, take out all the enemies using the Riot Suppressor ammo. Head inside on foot and use the switch on the upper level to lower the blockade and bring in the Batmobile.
Now use the Battle mode of Batmobile and fire the Power Winch at the anchor point to create an opening. Head inside and fight the goons. You will be fighting alone so it is wise to start with Fear Multi-Takedown and defeat all the enemies before destroying the weapon cache.
At this point, Penguin will reveal that he has captured Nightwing and he will be pissed as you have already taken down 80% of his operation.
Weapons Cache #5 – AmerTek Building (Founders’ Island)
You will receive Nightwing’s location from Alfred. Head over to the east side of Otisburg to find the AmerTek Building. You need to make your way to the “Opening Soon” sign on the west side of the roof.
The rooftop will be crawling with enemies that you need to take out. Make sure to take out the sniper first and then the thug with the jammer to make the fight a bit easier.
Once you have dealt with all the enemies on the rooftop, use the switch panel next to the elevator shaft to gain access to the rest of the building. Drop down the elevator shaft and you will see some thugs below you holding Nightwing captive.
Crouch and enter the vent behind you. Use the Remote Hacking Device to disable the Sentry gun and then enter the vent in front of you to get behind Nightwing. Exit the grate and press the button prompt to rescue Nightwing. This will trigger another tag team fight sequence.
Defeat all the thugs and enter the vault to use Explosive Gel on Penguin’s last remaining weapons cache. Now when you close the vault door, you will find Penguin holding Nightwing at gunpoint, AGAIN.
Luckily you can simply use the Dual Team Takedown to beat Penguin and detonate the weapons cache in front of him. Now all that is left is to take Penguin, the Gunrunner, to GCPD and lock him up in Batman: Arkham Knight.
