"As Freya, kill an enemy god using Valkyrie's Discretion while you are at 15% health or less"
You're ult will make you fly in the air and allow you (while untargetable) to shoot some magic balls to your enemies dealing splash damage.
Don't forget it has a certain time to be off the grid (so you can die while casting it) and that dot damage still affect you.
So the trick is to focus an enemy and bring him to low health, and when your life bar reach the 10% (so basically red part), you ult to finish him down.