Due to their flamethrowers Batman was put on the defense but he quickly threw a pair of pair batarangs to jam the flame throwers.
After singing them a lullaby, Batman quickly pulled out his communicator to seek Batgirl's advice on how to close the pipes. "Batgirl, I need a way to shut off the gas works."
"Sounds like you found the Joker." Batgirl surmised.
"He's going to pump Joker toxin and Promethium through the abandoned gas pipes under the city. Gotham will burn to the ground, choking on its own laughter." Batman answered.
"Hmm…There should be an emergency shut off switch on the roof. A set of three wheels just like in the basement. If you turn all three of them that should reverse the water flow." Batgirl answered.
"Right." Batman said before entering the flaming door carefully and dropped down hole in the floor while avoiding the flames.
He had to snub three stooges that got in his way and headed straight for the opening slightly left of center. He followed the path and entered the coop-wire entrance. Unfortunately the Joker was standing above the door to where the shut-off valve was located.
"Ah, Batsy old chum. Back again?" Joker asked as a rather mime walked up with Harley on his back, only the way he was walking looked more like he was stumbling. "You can't erase all your hard work now. It's all flames and giggles from here on out."
The mime had a large jet pack strapped to his back and attached to that jetpack was what appeared to be missile launchers.
"Especially for you, Bats." Harley said, hopping off the large mime's back. "Show him what for, Fatty."
The so-called Fatty knelt forward as two panels emerged from his back, revealing two more missile launchers. Batman quickly rolled behind a stone pillar and pulled out his communicator.
"Batman, you need to reverse the gasworks pipes now." Batgirl spoke. "There's nothing we can do once the gas gets out into the city."
"The shut-off valve is blocked by five hundred pounds of clown and twice that in explosives." Batman replied.
"If you can work your way around the side, you can get in through the back." Batgirl advised.
Batman put the communicator away and followed Batgirl's advice, going back through the exit gate and entered the door to your left side. There was another thorn blocking the door to the valves. He had to run past one bozo, but that didn't take long. Beyond the door was a series of wheels like in the basement with the method being similar to the green to green (activate) puzzle only now it's red on red (deactivate).
Batman turned all the wheels, watched as the pipes realigned themselves and then contacted Batgirl for an update.
"It worked! You've completely reversed the gas flow. Now the Joker's only supply of toxin is what he has in the blimp."
Batman then exited the opposite door and climbed the ladder. He made quick work of two stooges near the exit door, making sure to conserve his energy as he made his way back outside. Outside, the Joker was all red after the Dark Knight's stunt , noticing that the pump he attached to the Gasworks from the blimp was reversing the flow of toxins.
Looking down at the newly emerged Caped Crusader Joker was quick to make his disappointment known. "No, no, no! No fair!"
It just couldn't be easy for once, could it?
"Somebody kill the Bat!" Joker said to his two assistants, Fatty and Harley. "I'll take care of Gotham, myself."
He quickly grabbed the ladder as it retracted back into the blimp, making his escape. He lets his assistants kept Batman company while he made his getaway. Fatty's jetpack carried him and Harley down to face Batman. Harley hopped off and took a seat on the center pedestal, content to watch the oncoming fight.
Fatty was as his name described: a fat looking thug but was heavily armed with rocket launchers and even had flame throwers for close range.
So Batman resorted to long range combat. First, he had disable Fatty's rocket launchers. He started by targeting those which are ready to fire which showed a literal red light. Of course Batman made sure himself keep a safe distance since Fatty would utilize flame throwers if he got near him. Also, he had to avoid the thug's depth charges, those explosives that project instead of going directly at him.
One last thing, he also fires heat seekers whenever Batman got him by hitting a rocket before it was fired.
The best way to avoid this was to stay behind the concrete posts and strafed to lock onto the target. Of course, when they had already been fired, Batman needed to outrun them. While Fatty was heavily armed he wasn't durable since Batman only hit him four times and he was down for the count. He bumps the towering pedestal where Harley was, knocking her down.
"Aah!" Harley cried as she fell onto the ground.
Fatty was still stumbling with a dazed look on his face as Harley sat up and glared at him. Only a second later she let out a cry when she saw what was about to happen and had no time to move as Fatty fell forward and crushed her with his entire weight.
Batman had rushed over but saw Harley was fine and was likely not getting up any time soon.
Sweet retribution!
To use an age old aphorism, it was like hitting two birds with one stone!
Batman had no time to think any more on it as the Joker's blimp began taking off. Batman quickly rushed after it, firing his grappling gun before it was too late.