This is the first achievement you can pick up in the game and is missable, so follow the guide carefully.
During the Prologue, you’ll be given a spyglass and asked to mark Count Etcheverry. When you gain control of the spyglass, move the center crosshair over the person on the left of the tower opposite you and press "A."
If executed correctly, Geralt will say "one and a half degrees," the Ballista will destroy your enemies and your journal will be updated with Count Etcheverry’s info. If that doesn’t happen then re-load your last auto-save and try again, you will not get the achievement.
The achievement will eventually pop after the entire sequence finishes, by which point the auto-save will have shifted to a later point.