Just wanted to post my findings as a player of this game for 17 years.

Scarecrow: To my knowledge, unlike the other bosses Scarecrow doesn't change with difficulty, other than his health. Just stand in the middle of the room, back a bit, and keep the camera zoomed out for maximum coverage. Look for his trail while moving as little as possible, and try to attack a tiny bit ahead of where he appears to be.

Rolling is the fastest way to take out ghosts; in my opinion, it's not worth it to try and use them to build up meter with combos. If they drop red powerups, take them; that's the best you'll get out of them. Also, for the love of god be careful not to roll/move too much, disrupting the smoke and making it harder to spot Scarecrow during the ghost phase.

When he becomes visible again, make sure to build up meter and go for a full bar before he goes away again. He has started up the ghosts again for me in 2 ways: He's been knocked down too many times, or he has lost approximately 50% of his health. Haven't ever had him start up ghosts again from being idle. Currently I can 2-cycle him with Batman/Robin on normal.

Clayface: Vulnerability windows are timed, not tied to losing a certain % of health.

1st Phase (Blades) changes attack frequency and speed with difficulty. On Dark Knight, Blades are incredibly precise to dodge, moreso if using Robin/Batgirl as their rolls have less distance/speed.

Has an attack during first phase (Blades) that he does only if you get too close (About 3 Batmans away). It is programmed to hit you no matter what, unless you roll at just the right time. Given the batarang cooldown being currenctly unavoidable, you can alternate throwing a Batarang at the conduits, and then running up to him and rolling away. This will more or less cover the cooldown without ever letting him do the actual blade attack, which is lengthier and harder to dodge while still batarang-ing the conduits.

When he is made vulnerable from the electricity the first time, a cutscene plays and the player can't move. All subsequent times this happens, you can run right up to him for positioning and even start hitting him, the electricity can't hurt you.

2nd Phase (Blocks)

Pretty straightforward. Blocks can be dodged without rolling on Normal or lower. Haven't found a way to skip this phase you you just have to wait.

3rd Phase (Mooks)

Unlike other floor blobs that spawn Clayface goons, the ones Clayface tosses can't hurt you at all, either from the projectile thrown or standing in them after they've landed. Stand near a ledge and let him toss all 3 at you, overlapping them; then, use either throws, aerial attacks or basic knockdown moves (punch x3 / kick x3) to knock goons over the edge for quick kills.

Bane:

The 'Bane' of my existence, more like. Vulnerability windows are completely timed, not tied to taking a certain % of damage like Scarecrow.

1st Phase (Purple Health Meter) requires more precise rolling to dodge/stagger him. This is exasperated with Robin/Batgirl because their rolls are slower/cover less distance. You do NOT need him to hit a wall during this phase to stagger, only a successfully timed roll is required to stagger him and make him vulnerable. After research, I've found that the angle is important too... don't try rolling directly towards (duh) or away from him... the former gets your face punched, the latter causes him not to punch at all.

After analyzing P1ZZAC4T's run, it's clear that the grapple is a viable alternative if you don't feel comfortable rolling. I've found that the most consistent method is to roll perpendicular to his attack; this strat is borderline mandatory for Robin/Batgirl, but this is fine since they accumulate so many more extra points that you won't have to sacrifice anything to have grapple in the first place.

For Batman/Nightwing, I'd say if you buy grapple, save Flash Pellets for your last when you get to Sin Tzu's stages.

2nd Phase (Red Health Meter) requires a successful dodge and for him to hit a wall in order for him to become vulnerable. Ground Pound / Falling Stalactites attack is unavoidable to my knowledge, but if you get health/red orbs during, it's helpful.

UPDATE: You can run up to bane, punch him once and roll away, and he will start approaching you again early. Be forewarned, however... it's up to you to keep moving and not get knocked down by a stalactite... even just one hit can make re-activating Bane early near-pointless.

Sin Tzu:

Has the annoying trait of only being vulnerable to charged attacks. Always try to have a full meter when he's not hiding/spawning goons. Health also recharges during this time, so finish thugs ASAP.

Is programmed to spawn 4 waves of 4 goons (2 regular, 2 gold statues). Seems to have a % requirement before he runs away, might be able to abuse this by getting him right to the threshold and then doing as much damage as possible to bypass a wave of goons at the end.

Phase 1 (Him standing there)

You are intended to continuously approach him as he runs away until he attacks, but you can get him to attack you immediately with a taunt (in taunt range). I've found that rolling in a triangle is the most reliable way to dodge his strikes; with Batman/Nightwing you can just mash roll for proper timing, with Robin/Batgirl you need to watch less than 1/4 of a second between rolls to ensure you pace yourself with his strikes. MAKE SURE to stay away from him during his backflip animation after he strikes 3 times, otherwise you will lose your chance to hit him.