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The Joker Boss fight at the end of the game is very hard on NG+. You have a titan thug, Mr. Hammer, a bunch of other thugs with knives and tazors, and rollercoaster carts zooming by. It can get very hard, but I have the solution.
When Mr. Hammer and the titan thug come out try to avoid them. You want to start thinning out the other thugs. Always move, if you stay in the same place for a long time they will surround you. When you get to a group of the thugs counter them so they separate. Then you want to focus on taking out one thug on his own. If other thugs come over when your performing a ground take down, throw batarangs at them to stall them. Avoid hitting Joker because he will just counter your hit, breaking your combo. When you finish with the thugs, start with the titan thug. When you see Mr. Hammer, stay in one place until he jumps, then dodge to the side. He will then stay in one place for a little while. After you deal with the Titan thug, Mr. Hammer should be easy. If your low on health keep your distance and play it safe.
Well that's all I can say. Good Luck!
Another simple method of beating them is to stun Mr. Hammer/Titan Thug as soon as you can. Start attacking and use your bat swarm as soon as you can. Continue to attack and swarm, dodging when necessary, and you should take Mr. Hammer or the Titan down in no time. If you do mount the Titan, go for the thugs, because they're easier to get rid of and make harassing Mr. Hammer much easier. Power gadgets also help immensely if you can build up a combo large enough, specifically the quick gel explosives.
Another way I found made this easy was to jump over your enemies constantly. For this you should attack Mr. Hammer first and punch him a bunch of times then jump over him after about 3 seconds. Repeat this process until you beat him. If you do this right you should beat Mr. Hammer without losing any health. Now just try to beat all the thugs. Use the same strategy as before. Just punch a guy, punch another and jump over one. The titans will not be able to hurt you if you do this. After you beat the thugs beat the titans what ever way you feel is easiest.