Without spoiler stuff.
After you complete the quest ''Touching the Sky'' you will get the Ariel Bow.
With that some elven arrows and blooded arrows.
- Equip the Ariel bow with the blooded elven arrows
- Go outside in Skyrim and wait till the sun is up.
- Shoot the blooded elven arrow into the sun.
- And Voila the trophy is yours.
p.s. It doenst work at night time