When we've had a break in the middle of the raid I've invis'd over to him. Then you can stand next to the stairs wall, about halfway up to snag him. This is not easy.
I usually wait till we have cleared some of the trash near him, and then slip over to the wall to use him to clear the rest of the nearby trash.
That said, I think the assassin mobs are the second best in this zone. Look for a poison proc icon. SK's are almost as good though, and quite common.