Make sure you have at least one cleric to res, and make sure that cleric stays alive. Your tankers will be losing a lot of exp... Cerberus 3 quest does NOT spawn mobs, so you dont have to worry about that. It's just that the guy takes so long to kill.
In order for your party to go with you to kill Cerberus, each party member has to have that exact quest. If they dont, they dont get teleported with you to Cerberus.
No cleric
With 1 cleric
I started off with 42% exp, went all the way down to 8%. When we finally killed him, I got back up to 44%.