How are skulls awarded?
Solution 1:
Rift points determine 4 of the 5 skulls, but the 5th skull is dependent on the amount of time it took you to finish the level. You can check the time it took you and the par time in the statistics at the end of the level.
You also must be playing on at least War Mage to get all 5 skulls. At the lowest difficulty you will get far fewer.
If you're aiming for 5 skulls, there are many video guides on YouTube, and I also found this Steam forum thread.