Usually at castle you'll have a group of them chasing you in a circle and turning back to hit them every so often is very affective.

To be honest when I'm at mod I get on average 5 kills before being killed and at castle I get about 10 before being killed. I think I get less at mod because there is that narrow doorway at mod where it makes it unavoidable from getting hit.

There has been times where I've gotten 40 kills at castle before being killed.