Shadow the Hedgehog

On the missions menu/map there are dots on the far right they represent bosses depending on the path you took the different bosses you'll fight,beat all the bosses for a new big boss.

Do hero and dark missions to go to other levels

Basically, just play through the game again. You can play through this one with variety anytime you please.

Like other Sonic franchise games, this has a "Final Story" feature that you must unlock. On the right on the story
map you will find a row of circles. These represent end bosses. Each time you beat one you must start a new
story and trace a path to a new one. Some are Eggman, some are the General, and some are the leader of the
aliens. Once you defeat them all, you can play through the final level where the true final boss is revealed.
It is not neccessary to complete every mission, only to beat all the end bosses (I always avoid Edge City because
I can never find all the bombs in time.)