Successfully complete the game under the easy difficulty setting.
Scarecrow is a master of fear, and can create a chemical to heighten it.
Actually, in this battle, he has very few attacks. You start off in a gas-filled room. You will see ghost-like things flying around. ONe hit will kill them. Look at the blue gas surrounding the room. Notice that the gas seems to part in some areas, and you can hear footsteps. Run around, rolling where the gas clears.
If you are lucky, Scarecrow (who was invisible), will appear.
Then, beat him up and repeat.
Scarecrow is fairly easy, because he rarely attacks. To start off, look down at the floor. It is covered with some sort of blue gas. The trick to defeating him is as follows. First, avoid or destroy all of the ghosts flying around.
Notice how the gas seems to part in certain areas (byt not your own). This is Scarecrow walking. Roll into him. He is invisible, and this lets you see and beat him up. After awhile, he turns invisible again. Repeat the steps until he is defeated.
This Boss is made of clay, and can be difficult. He has several attacks. If you get too close, he makes a brick wall out of his chest then rams it at you. He also can turn his arms into sledge hammers of razor blades and lunge at you. The trick here is to dodge until the lights turn green on the pipe-like electricity things on either side. Then, throw a Batarang at both as fast as you can, or hit them. The electricity will activate, and Clayface will get shocked. Go up and attack him. When he gets back, run to the far side. He will probably launch some kind of muck thing at you, which produce more muck monsters. Beat them up, then repeat step two. After a few hits of electricity, dodging the attacks, you will win.
Clayface can shoot muck which spawns little muck monsters, turn his arms into sledgehammers/razor blades, or ram you with a brick wall if you get too close. The trick is in the two electric outposts-like things on either side. Dodge until they turn green. Then, quickly hit each with a Batarang or your fists. This will turn on the electricity. He will be stunned. Hit Clayface's face and repeat this until he is defeated.
Bane is very strong, but lacks IQ. Unfortunately, he starts off invincible to your attacks. Stand back and wait for him to charge. Dodge, and he will be off balance.
Then, beat him up. He will do the same, except now you have to make him run into a wall, hitting him. Unfortunately, this causes rocks to fall from the ceiling. Dodge them. Also, he will start using some kind of earthquake attack -- stay back if he is not advancing on you. After you beat him up, he will heal. Repeat the previous steps.
Bane starts off invincible to any attack. Avoid him until he rams you. Dodge him to put him off balance, then strike. Afterwards, you must make him ram into a wall.
After beating him up, he will stomp to make strange boulders fall from the ceiling.
After those attacks are dodged, repeat. After you knock out his health, he returns stronger. Repeat the previous steps to defeat him.
Defeating Sin Tzu:
Sin Tzu is immune to non-special attacks, and is very fast and powerful. Start by watching him run around. He will go into a series of four rushes. Dodge all four of them, and he will backflip. Hit him then. Repeat, but do not stay around to long, until you get a full power meter. When he backflips, hit him with a power attack.
Do this for awhile. He will then run into his tower and summon enemies. Beat them up. Meanwhile he will be recovering health and shooting fire at you. After defeating his allies, get your power meter back up and repeat the previous steps. He will now occasionally go into a super-nova attack. Block it at all costs and repeat until he is defeated.
Sin Tzu is blindingly fast, amazingly strong, and invincible to any non-Power Attack.
Start by letting him run at you. He will do a four-hit dash. Dodge all times and he will do a backflip. Hit him. Repeat this until your Power Meter is full. Then when he backflips, hit him hard. After a few attacks, he will jump into his tower.
He will summon units, regain health, and shoot fire at you. After you defeat the enemies, he will stop healing and fight you. This part is all the same, except he will occasionally move to the room's center and do a crazy attack. The only hope to dodge this is to block.