Actually you would NOT get a Look Out Sir! roll. According to the rules on page 99 of the main rulebook, LOS! applies to shooting attacks that use templates, but not to shooting attacks that use Ballistic Skill to hit. In addition, according to the Character rules on page 99 and the Bolt Thrower Rules on page 111, the Slann cannot be hit by the Bolt Thrower so longer as there are 5 or more rank-and-file (not champion) Temple Guard in the unit. In that case, you just act as if the Slann isn't there (unless it is alone in the second rank as discussed below). Since both Slann and Temple Guard are Infantry (albeit different bases sizes), the Slann can only be hit if there are less than 5 rank-and-file left. If there are less than 5 rank-and-file left, according to the Bolt Thrower rules on page 111, at the point when the bolt hits the rank the Slann is in, "it will be necessary to randomize which model in the rank is hit." The question not clearly answered by all this is the question of when you count the rank-and-file models to see if the Slann can be hit. I think the best read is that you count at the time the bolt reaches the rank the Slann is in. If there were 5 rank-and-file but one in the first rank got killed, then the Slann could be hit when the bolt gets to the second rank. In addition, the language on page 111 under the heading "Bolt Throwers and Characters) appears to indicate that a Slann that is alone in the second rank of a Temple Guard unit could be hit by a bolt thrower even if there are 5 or more rank-and-file models left at the time since it is in a rank made up entirely of characters.