I've never seen anyoone take on all 4 towers plus drek or vann. I ahve seen back in BC a tank in full T6 take on 3 but that was with 8 healers. Again not saying it's impossible but I think it's highly improbable for a 5 man team to sneak in and take on Vann or Drek. The more people you bring the more of a crowd it draws and then you will be facing the allies as well. Waiting for 2 of the towers to go down is fine but at that point you should have more teammates around by then anyways.