SONIC UNLEASHED

Hello. Here are all the guide videos I found to help you get through these medals on all stages. They will be in order going down from "Apotos" "Spangonia" "Mazuri" "Holoska" "Chun-nan" "Shamar" "Empire City" "Adabat" then the boss battles for last. If you're still missing medals, then make sure you do all the mini missions and get the medals which isn't that hard, that and go to the professor and give him all the Souvenirs for medals.
Credit go's to SonicTheMedals for the videos.
Happy Hunting ~

To add to the previous contributor's good *general* guideline on how to get all the sun medals, I have some more specific details:

This Gamefaqs walkthrough by Sonictrainer has pretty much every collectable listed in the game: http://www.gamefaqs.com/xbox360/945571-sonic-unleashed/faqs/...

For more visual guides, there are some great sun and moon medal guides on Youtube. I have yet to find a user who has done a video for every single medal however, but videos for all levels do exist and they can be found pretty easily (i.e. "sonic unleashed [level/area name] medals" in the search bar)

Also note that there are a few rogue sun and moon medals located in missions given in the hub world. Even more annonyingly, there are some medals located in missions given after you have completed another incredibly difficult mission first. So this achievement is quite a bit more difficult than it seems. This is where Sonictrainer's guide comes in handy to find those medals, since videos that detail where mission medals are tend to be a bit more scarce.

I recommend going through all the level and hub medals first, then when you have all of those, check via the world map which worlds still have medals than need collecting. Then use Sonictrainer's guide as reference to find those last few medals.

Here is a fantastic set of videos
http://www.youtube.com/playlist?list=PLB2182DD264D3E107

They cover all the day and night videos.

All credit goes to the creator of the videos ShadnicK826

Hope this helps any one after these medals.