I was playing with a ranger hero the other day and thinking about the warding ability. Sure, it is nice that it gives some defense against the dreaded ambush, but if your opponent has no ambush then it does nothing at all. I would prefer an ability that gives some protection against ambush, but that isn't irrelevant otherwise.
What I came up with is an ability I call 'healing' (it seems like a good fit). Healing is a percent chance of stopping a killing blow against one of your units. I use the term killing blow to encompass both insta-death from ambush and a unit losing its final pip of life in normal combat. Each unit could only be healed once per battle.
I'm not sure If I stated that very clearly. But 100% healing would essentially add an additional pip of life to every unit in that hero's army and be complete protection from ambush (essentially causing a successful ambush to take that additional wound). Of course 100% healing should be impossible to attain. So, perhaps 50% healing would be a more sensible figure to look at. With 50% healing a unit could be protected from ambush half of the time, OR have a 50% chance to survive having its final point of life taken in combat. One of these outcomes could occur once per unit in each battle.