My DK and 4 Shammys do really well in AV. Just dont get to close to the enemy and chain lightning alot. And everyone will love you when ever your healing the tank no matter what your spec is. Or when you chain heal a group people infront of you.

As far as tower defense I generally spread my guys out inside the tower as to not keep them so close that an AOE attack kills them all. It works pretty well for me and if I get over whelmed i just release my elementals.

I even did WSG today. I kinda felt bad for the other team because all i really did was guard our flag. They did get a single one from me. Down side was the match lasted all 25 min.

Bottom line is 5 of anything I think would be good in AV. Its a large enough group that most people will probably not hate you for taking up space.