Surgical Strike Exhausts to add a damage and strain to a hero after an imperial attack.

Does this card ready during the status phase or is it once per mission?

I thought I read that it readies during the status phase but I can't find that now.

Rules Reference Guide p.12

Many game effects and abilities can require a player to exhaust

a card or token. Cards and tokens that are exhausted must be

readied before they can be used again.

Rules Reference Guide p.23

Ready: The Imperial player readies all of his exhausted Class

and Deployment cards. Each hero also readies all of his

activation tokens by flipping them green-side faceup.

-- Hero Class cards and Item cards are readied at the start of

that hero’s activation, not during the Status Phase.

So Imperial class cards get readied in the status phase (before deployment but after threat increase).

Hero class cards get readied at the start of the hero's activation.

Awesome sauce this is my new favorite DS ability card.