Streets of Rage 2 - Stage 5 boss R.Bear. Cheap, massive and the fastest obese man in video game history. Can also randomly body-splash you ass-first from anywhere on the screen. Every other boss in the game can be beaten with skill - including Shiva and Mr. X - but this twat requires cheesing tactics almost exclusively
The real spike, at least on higher difficulties, is the elevator in (I think) stage 7. Cheesing isn't much help there. That part alone makes me think that Mania difficulty is something they threw in for a laugh without testing it.
So the thing about the SOR2 bosses is that, for the most part, they follow a pattern, and the higher the difficulty, the less likely they will follow that patern and go rouge,
The main rule to consider is that the key to getting little to no damage from them is to back off and let them cool off after they've been knocked out. They have an "instant recover and fuck shit up" mode that is activated upon being knocked down. There ARE ways to circumvent this (like throwing a grunt at them when they wake up, or finding that one frame where they AREN'T invincible), but like all Beat'em Ups, the CPU suffers from really stinky cheese, and unless you play like the Brazils (in which they go out of their way to use glitched to cheese right back at the CPU... MAX always seems to be their go-to character for that kind of stuff), you ain't going to get very far if you're aggressively attacking.
Boss 1 - The only one that you can get close most of the time without any real reprecussions. Side-walk to him instead of walking TOWARDS him. 50% chance he'll grab you and throw you, but after that throw (and once you recover from said throw with holding up and the jump button) he's fair game and won't attack until a certain amount of seconds have passed.
Boss 2 - This one requires you to stay completely on the opposide side of the screen. What wil happen here is that he will ALWAYS wind up using the straight horizontal jet attack, which you can spam jump attack, and repeat the pattern until death. This is assuming you HAVEN'T triggered the grunts and just tip-toed to activate the boss ONLY and not the grunts. (this is one way you can cheese the game)
Boss 3 - He's a very jumpy boss, but after like 2 or 3 jumps, he wil go for an attack. If he Blanka spins, jump attack. If he runs towards you, attack him with whatever you want to. You do NOT want to be next to this guy after he's knocked down because he will IMMEDIATELY uppercut you, but you cal also use this to your advantage. If you're within a certain range of pixels near him to activate his uppercut but enough away from him where you can walk away without taking damage, you can fuck him up AFTER the uppercut AND activate a loop of him doing that until he dies (or he wises up, whichever comes first)
Boss 4 - Like boss 2, keep to the other side of the screen. You WANT him to activate the running clothesline because it's the easiest way to attack him believe it or not (you can jump attack him or dash attack him). Geting too close means eating a german suplex. make sure you do big single attacks because doing a string of attacks will activate his counter attack. Trying to throw him will ALSO activate the counter-attack.
Boss 5 - Make him splash and get the fuck out of it. He's vulnerable after he lands. Harder diffciulties makes him fast as fuck and not splash as much.
Boss 6 is just boss 2 and 3 combined, you wanna take out the flying boss first BEFORE you go for blanka.
Boss 7 - Quite posibly the EASIEST boss in the game outside of Boss 1 and 2. All they do is jump'n'shoot. if you're using MAX, you can just slide all day and get them off-screen and kill them that way.
Shiva - DON'T GET CLOSE!! You have to bait this motherfucker into using Final Crash while avoiding it (by walking away), and unfortunately he doesn't use it often. You'll know immediately when he's about to use it when he walks up and down, then he wil run at you to Final Crash your ass. Sometimes the up'n'down walk is a 50-50 that he doesn't do shit though, so it's all about patience. You might run out the timer in this battle.
Mr.X - Yea this one sucks, do your best to avoid the bullets, which can easily be done if you're in the middle of throwing a grunt. He can be attacked after the bullets.