Make sure you have the Rocket Launcher and the Broken Butterfly Magnum from the castle. Also, have at least six magnum rounds. When the first Chainsaw Ganado appears, shoot the Rocket Launcher at him before the other Ganados get in the way. For the second one, pump all your Magnum rounds into him. If that does not kill him, then use your Striker or Riot Gun. Note: You might get damaged during all of this; make sure that you have at least three First Aid Sprays, and two Herb Mixes (Green, Red, or Green, Red, Yellow) to use if your health gets too low.