Bombshell Bill

Bombshell Bills are golden relatives of the Bullet Bills fired by B. Bill Blasters. Bombshell Bills are much stronger than Bullet Bills and are positioned so as to guard extremely important areas.

Paper Mario
In Paper Mario, Bombshell Bills appear in Bowser's Castle and are a dark yellow. They have low health but high attack power and are spawned once per turn by each B. Bill Blaster present. While strong, they are highly susceptible to moves such as Air Lift and Hurricane that can remove them from battle immediately. Like the game's normal Bullet Bills, they are instantly defeated after they attack, even if they miss or are guarded against.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Bombshell Bills appear in the Palace of Shadow and are a much brighter yellow. They are still highly susceptible to being blown away as well as several status effects, but they now have higher defense. Superguarding can now also be used to defeat them instantly when they attack.

Trivia

 * Unused sprites for a similar golden Bullet Bill are present in Mario & Luigi: Superstar Saga. Its Bill Blaster would have been raised compared to its normal variant and have a longer attack charge animation.