This may not be exactly as you want it, because I think I'm trying to do something slightly different from you, but here is something I've done. I think it should provide a solid basis for you to tinker around with.
First I created my 'resurrection' skill. I have set it (via the damage formula) to revive with 10% of HP. If you want a different percentage change the 10 in the formula to what you want. You will see that the skill is set to 'Never' as the character, obviously, shouldn't be able to use it her/himself.
Then you need the troop event. I'm using a script so that I only have to create one troop event and it runs in every battle. I referred to it in an earlier post. If you don't want to use a script you will have to put this into a page on every troop you create.
The way I have set it up, it happens every time the character dies in a battle. If you only want it to happen once per battle, you will have to play around with the turn settings. Or maybe use a variable set to random so that there is a one in 3 chance that it will work? Or change the 100% success rate to something else? For the sake of clarity I'll just mention that 'Unconscious' is what I've named the death state.
Hope this helps.