A 494:
Dismissal couldn't be used against a swarm, because (from the swarm traits), and Dismissal targets a single creature. Banishment, however, can target any number of creatures, so long as they're close together, and so this restriction does not apply. It does have a maximum HD for its target, but the swarm traits don't care about that (and any swarm is likely to be under that limit).A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate)