The Joker: You should fight the Joker but don't. He's always got some dirty trick up his sleeve and he's smarter than you think. Even if you win and don't die, he'll probably come back with some horrific retaliation against you and your family.
Poison Ivy: Don't fight Poison Ivy. She might not be the best hand to hand but she's got all those strangling vines and giant venus fly traps to feed you to. If you win, you're never going to be able to leave your house again and if you have a garden, you're fucked.
The Riddler: You should absolutely fight Riddler. He's an arrogant, patronizing nerd who thinks he can get away with talking smack. Fight him and shove him in a locker. Asshole.
Mr Freeze: I mean, you can fight Freeze but why would you want to? He's done some bad things but give the guy a break, okay? He's just trying to help his wife. Don't be a douchebag.
Catwoman: Why would you want to fight Catwoman? She's just here for your stuff, which is probably insured. She's got a lot of hungry kitty mouths to feed, okay? Just calm down. Plus, she's x10 a better fighter than you'll ever be. Don't be fooled by the tight-fitting spandex.
Scarecrow: Fight him, so long as you have a gas mask. He's an asshole. He got his "violent dancing" but it's 99% most likely bullshit.
Two-Face: Oh yeah, fight the scarred guy. Does he deserve it? Well, yes. I mean, he beat up the first Robin with a baseball bat and has killed a bunch of people but his life is HORRIBLE. Do you know how hard it is to find a date looking the way he does? He'd probably just shoot you before you could land a punch, anyway.
Bane: Don't fight Bane. It's not a good move, ask Batman.
Harley Quinn: Lots of people have tried and failed to fight Harley Quinn. Why would you want to? She's so adorable. And she's got enough on her plate as it is. She'd probably kick your ass but give the girl a break, jeez.
Ra's al Ghul: DON'T FUCKING FIGHT RA'S AL FUCKING GHUL. A FEW WEEKS OF TRAINING WILL NOT BEAT AN IMMORTAL ASSASSIN WHO IS THE LEADER OF A LEAGUE OF THEM. LEAVE RA'S TO THE PROFESSIONALS.
The Penguin: Fight Penguin. He's a pompous prick who needs to be taught a lesson. Watch out for the sharp end of that umbrella, though.
Man-Bat: Don't fight Man-Bat. Poor guy can't control it. Just leave him alone.
Killer Croc: What the fuck is wrong with you? Don't fight Croc. Dude eats people, which should be motivation enough to stay away from him but apart from that he's a chill guy. He gives people a home in his sewers. It's fine, don't fight him.