I don't think archons and succubi (and indeed any eldar characters) will ever be as deadly as they should be until they finally make WS matter as much as it should. Make it like the 'to wound' chart, where every point higher or lower is a significant change. For example, a space marine captain would hit an archon on 5+, a succubus on 6+. By the same token a kabalite warrior would have a very hard time hitting the captain, while an Avatar of Khaine or Bloodthirster would laugh at all of them.
You would have to have a serious points cost overhaul of course, but you would end up with characters that actually do use skill rather than heavy armour to win, like they're supposed to. Taking a shadow field and hoping it doesn't fail is not a satisfactory solution. Eldar characters should be expensive character and utterly deadly, especially in one-on-one fights. As it is, even a very basic space marine captain is going to beat one, AP2 or not.