Collect all Art, Music, Secret Documents, and Movies except for #36 to unlock the Final Adabat mission.
Earn an "S" rank on all missions to unlock the final mission. Play the final mission, and complete it with all hidden items collected to unlock the "Sonic Omake" sequence.
To get more points for Sonic the Werehog (Dark Sonic), just destroy everything you can find. Look at the lower right of the screen to point yourself in the direction where hidden objects are located.
Always talk to the townsfolk/villagers more than once. They might have an unlockable item for you.
In a day level, press Circle during the starting countdown between "1" and "Go". If done correctly, Sonic will start with a Spindash boost.
When battling Dr. Eggman's first robot, Beetle, collect the coins so you can dash him when he tries to pincer you. This fight consists of him releasing three boxes from a distance (just run around them), then moving in close to pincer you. Jump then dash in to him when this happens. Once he is down to a certain amount of health, a side view will appear. He will start on the right-hand side and fire three missiles. The pattern of how they come at you depends on how their engines fire up; watch closely. He will then move to the middle of the screen. He will pincer attack once or twice. Just dodge them. Finally he will move to your right again and try to pincer you. Jump, then watch for the homing target, and give it to him.
You will first play as the Gaia Colossus against Dark Gaia. When Dark Gaia opens his hands, quickly hit him with two hands or let him swing his arms and dodge them, giving you a chance to hit him with a normal punch until you finish him off. Next, you will play as normal Sonic. Start running through the Gaia Colossus, and dodge Dark Gaia's tentacles and bullets. Run to the Speed Dash, and hit the correct combination. You must do this three times, each time for one eye. The levels get more difficult, so be careful. Finally, you will play as Super Sonic. Start collecting rings, and attack Complete Dark Gaia's eyes (seven in total). You will need at least eleven rings to attack an eye. Keep track of your ring count. Start hitting the side eyes, and dodging the rocks that are ahead, which get harder by the eye, until he opens his middle eye. Attack the eye, look all around for flying rocks, and hit the last eye. The score is an "S" or "C", so watch your time.
Use the following trick to dodge Gaia's six hand attack. While playing as Super Sonic in the final showdown against Dark Gaia, when he has all his small eyes open, he will try to slash you. A short time later he will get mad and attack with all six claws. It is possible to dodge this attack. Go just under the big eye in the middle of the slash. There is a small square you can fit in. You will not take any damage. However, if you get slashed, you will lose all your rings and fall into the lava.
The Egg Dragon is pretty easy to defeat. When he shoots ice balls at you, run away from them. When he shoots fire balls at you, jump or run away from them. When he comes in close to the rock field, jump and slice his power core (the light on his chest). Keep hitting it, then when he swings his sword later in the fight, double jump to avoid it. Finally, when he stabs the ground, jump.
When fighting aerial enemies as Sonic the Werehog (Dark Sonic), jump and attack once to bring them down. Then, unleash your fury on them.