I have played and beaten this game about 150 Bazillion times and found out that certain characters have different strengths.

Like Sonic is fast and everything else is ok and Knuckles is able to climb walls and attack is ok.

Also Tails is able to fly faster than Cream but Cream has stronger faster attack than anyone due to the fact of that flying thing thats with her.

So what I'm getting at is certain charaters work better than others.

If you are having trouble with the bosses here is some help.

The first boss you have to watch out for the hammer and hit the body. After you hit him a few times he will be destroyed.

The second boss watch out for the bombs and hit the cannon then when it is destroyed you hit the body 2 or 3 times then he is destroyed.

The third boss has those platforms that rotate... and have little gun things on them, take those out. Then you get on them and hit Dr.Eggman.
Then after a few hits he's down.

Then the forth one. He drops bombs, he has a platform that goes up and down. get on it when it is up then hit Eggman. Then after a few hits you destroyed another one of his inventions.

The fifth boss is a rotating wheel thing first you have to take out the cannon. then hit Eggman or knuckles if you haven't beat him before. Watch out for the hand. Then he's down another time. Were does he get all the money to build all these?

The sixth boss is quite hard, atleast I though he was. He has spinning platforms and spikey ones and guns. When they flash red don't go on them. When it isn't go on there and crush him. another one down

The seventh boss goes up and down. You have to hit him in the cockpit. My advice is change the gravity by pressing up and down go right under him and you got him.

The last level is all bosses. 1-7 and there's a final one. Wait by the platform closest to the left.Don't go on it, wait for the hands and jump on them and jump off. Then hit them a few times.

Wait till they are destroyed. Then when the one closest to the boss is right at the robots chest jump on it. Then on to his head a few times, then he is destroyed. Then you watch the credits and it says collect all the chaos emiralds.