How to handle damage in decisive attack when one party is unaware?

How do you handle decisive damage in cases where one party is totally unaware that they are being attacked and the other person has time to prepare?

The problem is that a prepared ambusher will not have had any withering attacks to increase their initiative prior to the ambush decisive attack. And it does not make a lot of sense to ambush someone with only your join battle initiative.

I’m looking for a reference to the rules explaining this situation, or a solution you have tried. personally all I could find in the book is that if you ambush someone they have a lower defence depending on whether or not they are in combat or totally unaware that they are in danger.

Just to make it clear what I’m trying to solve I’ll give two cases that came up in a game:

  1. Someone (a mortal) is sneaking up on a exalted that is sleeping and trying to kill that person. This is very unlikely (or maybe I don’t want it to) to outright kill the exalted, but how much damage does the mortal actually do?
  2. In a form of duel one party say, I’ll stand still and give you a first free shot. Do your best to hurt me. This is assuming there is only ever going to be one strike. This came up when someone wanted to test an armor.