In the 100% route, the standard powerup to enter this fight is double-spark, which is obtained near the end of 2-4. For most of Pix's patterns, the best strat would work with any double powerup, but there is one specific pattern where double-spark is invaluable.
The first minute is an autoscroller where Pix is invulnerable. Pix can move in two different ways: the lone red part can move clockwise or counter-clockwise. For the former, just stand still and duck underneath the red attack (left). For the remainder of the first phase, you can continue standing in the same spot. When Pix turns counter-clockwise (right), Kirby can stand in a different position a little off to the left.

After the platform stops elevating, the second phase begins and Pix is finally vulnerable. Each diamond requires four hits, but throwing a double powerup or an "atom" (the projectiles that spawn on the ground) of the same color does two damage. For both clockwise and counterclockwise patterns, there are three classes of attacks, and the clockwise attacks are roughly symmetric with the counterclockwise ones (though the colors of the atoms will be different).

In the first two attacks (left and middle), the double powerup is used to hit two parts of Pix for two damage, and the rest is matching atoms. Most importantly, the powerup is thrown in the opposite direction that Pix is moving in. Otherwise, it will only hit one of the enemies and the 1-cycle becomes much tougher. The first attack is always the same, and Pix hovers at a higher height (roughly Kirby's maximum jump height). The second, lower attack actually has multiple possible movements: the amount of time it takes for Pix to change direction can be many different values.

For the third attack (right), the three parts stick together and start spinning. Double-spark is used to get four hits in. I like to take damage after the third hit to knock myself back in time for the fourth hit, but this is not necessary.