1. The more outnumbered the heroes are, the more the enemies will miss.
2. The more impossible and desperate the shot, the more likely it will hit.
Watch Star Wars IV: A New Hope. The stormtroopers really can't hit the broad side of a barn, can they?
Gimli in Lord of the Rings III: Return of the King:
"Certainty of death. Small chance of success. What are we waiting for?"