Spike Pushers

This unnamed boss, nicknamed "Spike Shooters", is the boss of Wily Castle 1 in Mega Man 9. They are a series of small, stationary robots located at the ends of four "corridors". Each of these contain a large spiked ball the the machines continually shoot at, pushing them gradually to the left of the screen. There are two variations of the Spike Shooters. Blue Spike Shooters fire one bullet at a consistent rate, but only push the spike ball a short distance. Red Spike Shooters fire a charged shot less frequently, but pushes the spiked ball a much greater distance.

If either of the Spike Shooters push the spiked ball to the far left of the screen, the ball will retract slightly and it will no longer attempt to push it. Instead, they will aim for Mega Man with a small wave of energy. If Mega Man pushes the spiked ball into one of the Spike Shooters, it will be destroyed and a new one will spawn from the wall until the boss's health has been depleted enough.

Tornado Blow works well against them since it pushes back all the balls at once. However, chances are that the weapon energy will be depleted for that weapon, since it was used to operate the platforms earlier in the same stage. The weapon also uses a lot of energy, so you won't have enough to defeat the boss entirely with it.

Trivia

 * This boss is most similar to the Buebeam Trap from Mega Man 2, and are fairly unique when compared to other Mega Man bosses. They are both a number of stationary enemies arranged in a somewhat mazelike room where strategy and planning are more important than reflex and timing.  It is also somewhat similar to Atetemino from Mega Man 8, primarily in room design (though it is flipped on its side).
 * One of the boss' attacks resemble speed man's boomerangs except that they dont boomerang but just go in one direction.