A K Vikhagen
The moment you want to assign damage and not sooner (you lock the assigned damage to whatever is revealed on the flip, unless you are targeting other enemies as well) IIRC
EDIT: From the reference guide:
Facedown enemies can be engaged, targeted by effects, or attacked. If the enemy suffers [wound(s)], is engaged, or activates, that enemy is revealed and its card is flipped faceup.