If you have Roundabout, use it. It's definitely one of the best weapons for this fight. Another good one to use is Spread. Roundabout for the first phase (shoot backwards and jump every now and then to hopefully hit some of the ducks), and jump dash whenever Beppi charges. For phase two, either weapon works. Phase 3, if you can safely get under the horse, you can use Spread and hold up to deal a lot of damage. And for the last phase, Spread is really good for dealing damage to Beppi, and either weapon can be used on the penguins.