Out of focus, use at least 2-4 Steady Shots. (3-6 seconds)
Out of focus, pray they can't survive 100k damage over 10 seconds.
Realise a Frost DK hit you with howling blast and this whole 'one shot macro' is going to take 20 or so seconds of uninterrupted Line of Sight without being attacked. Lose, sigh, requeue.