Amatersu's power "Spiritual Renewal" should be fairly simple to apply. If a model takes a wound and she successfully channels this power and is in base to base contact, she may restore a wound. If she does this to a wounded multi-wound model it heals a wound. Same if she does it herself. If a trooper falls incapacitated (ie takes its last wound) and she has not activated yet that turn, she could restore a wound to it as well.
An incapacitated model that is not healed by the current turns conclusion is removed from play as a casualty. She may channel this power as often as she has actions to do so. Thus a two wound model that is incapacitated to 0 wounds could have Amaterasu spend one action to move into base contact, spend another action to channel this power and heal a wound, and spend her last action to channel it again to restore the second wound.