God of Light: Remastered

Every level has a total of 3 stars that you need to collect, which is done by completing the level while a beam of light is passing through each star. Some levels are practically impossible to finish without collecting all 3 stars, but many levels involve some more intricate reflecting of light to get all the stars than would otherwise be required.

Thankfully when you use the help of the firefly, it will always show you the level solution to get all 3 stars. So if you're ever struggling, you can use the firefly as a backup. It's highly unlikely you'll need to use the firefly for more than 10 or so of the 150 levels, since the majority of levels are incredibly easy and straightforward. But if you get stuck, use the firefly in small bursts to conserve its time. 

If you don't have any firefly time left, you can always consult the level solutions in the "Pure Light" achievement solution to see how to complete each level with 3 stars.

Each time you complete all 25 levels of a tree with three stars, you'll unlock the respective achievement. To see how many stars you have in each level, from the level select screen of a tree, right above each level number the stars you collected will be displayed, allowing you to replay any levels you missed 3 stars on.