SONIC UNLEASHED

Assuming that you have "Picking Up the Pieces" for saving the fourth continent this is fairly simple.

Return to Spagonia and talk to Prof Pickle.
After you talk to him and leave, you will automatically be sent to Shamar.
Once you get there go to Prof Pickles Shamar Lab. (if you are looking at the gate to the entrance stage then it is to your right)

Go inside the Entrance Stage and get the Light Speed Dash Shoes and clear Arid Sands Day Act 1
Go to Empire City during the Nighttime, enter the Entrance Stage, and clear Skyscraper Scamper Night Act 1

Go back to Shamar and talk to Prof Pickle Once you talk to him, Adabat will be accessible (although you don't need to go there yet)

Clear Arid Sands Night Act 1. Once you do that you can enter the Boss Gate for Dark Guardian from the Shamar Entrance Stage. Defeat the dark guardian and it will pop.