The Oathbreak achievement reads "While fighting Cernunnon and his oath-bound, banish all three of the spiritmasters' souls within five seconds of each other throughout the fight in Veteran Falkreath Hold.
Basically you just have to, on at least two occasions depending on your group DPS, kill these three bosses and carry their souls which appear as faintly lit circles to the glowing ritual pads.
Only one person can pick up one soul, so basically assign three players to three pads for them to bring the souls to.
Five seconds should be plenty of time especially with the strategy shown in the video below, by the way you can't stack the ice mage onto the other two archer and melee bosses; believe me I died trying!
You want to make sure you stack either the melee boss onto either of the ranged bosses and make sure all three die within relatively the same timespan so you have plenty of time to carry the souls to whichever pads without them reviving before the time is up.
Don't be confused when the achievement doesn't pop up during the fight as it will only pop up after the fight.