How is that one shot kill possible on Castor and Pollox?

First : Options 
Difficulty : Extra Easy
Save a crystal special, catch them with the crystal special during the backflip. Not to early they have invincibility frames. Not to late they get invincibility frames. To fight them land in the middle of the both use the Spin Special attack (Attack+Jump) to separate them, fight the greenish one using spin attacks to keep the red dressed one away from there tag team move.
Hope that helps :) game on

First Boss Kill

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