If you want truly broken, cry about avengers. They're incredibly tough, highly manoeuvrable, have loads of awesome powers and virtually never miss. They may not do tons of damage unless they crit, but they crit often and hardly ever missing cannot be underestimated.
I think the barbarian is just fine. I think the avenger is just fine. They do their jobs but do them in different ways and with different flavour. Like others have said, the barbarian is swingy with his damage output. I think you'll find that the averages will even things out.
Is it also possible that he's intelligently targeting low defence mobs? Are you putting a lot of brutes in?