The text of BT1's assassin trait says he becomes focused before he declares an attack, but I'm not completely sure how that interacts with his missile salvo ability. He's declaring an attack so I understand that he gets the benefit of assassin, but does he get it for each attack that is part of the salvo or is it limited to just one? And if its limited to just one does it have to be the first one that you roll or can it be the second or third? The confusion on my part is due to the text on missile salvo that specifies that you declare up to 3 attacks.

For all attacks.

Missile Salvo allows to perform up to 3 attacks with different targets. Before performing any attack BT1 becomes focused.