1. Refer to what Jerry said. Though similar, they are not exact synonyms.
2. The word to describe the scene is 'shootout'. A gunfight is much more focused and controlled, even possibly including strategy. The scene in this movie is to just shoot until nothing moves. Therefore, 'shootout'.
3. 'Shootout' without a hyphen. There is no change of meaning if you add a hyphen; it's just incorrect.