So, you've managed to reach these depths. I commend you. But your journey ends here, I'm afraid.
Although Cerberus might be the easiest of the shrine's bosses, he has several different attacks at his disposal. His physical attacks are not dangerous and he will use Thunderbolt occasionally, causing medium damage to the entire party. As with every other boss in the Earthgift Shrine, magic attacks do little damage. Despite its in-game bestiary input, Cerberus does not resist Sleep.
The party can easily defend against Cerberus's physical attack with an Invisira spell. Using Haste, Temper, and healing with magic users along with using physical attacks from the fighting characters should be plenty to defeat it. A Sleepra spell, cast by a Black Wizard, is also potent enough for the spell to stick.
Cerberus is a three-headed dog that guards the underworld's gates in Greek mythology.