Yes, we know that earning 100% earns you boss battle mode, but there is a small detail. You must fight a major big bad guy before you can acces Boss battle Mode. He is on the secret level, Dark Star which you unlock by getting 100%.

Anyway, the guy does not have a name that i know, so i call him Final Final Boss. To defeat Final Final Boss, ripple the fairy comes and gives you a sweet gun made out of the crystal shards.The only strategy is this one. Tab b button constantly.

Final Final Boss will counter your shots until he fires his attack. Shoot him while he shoots at you and he will bleed a little around his eye. After a few times that his eye bleeds, he rolls over and exposes his Halo. You must shoot that halo. Shoot it until it turns colors and gets smaller and dissapears.

Final Final Boss will then kneel over and produce a Cactus-like tail thingy. Shoot the tail, and it will flash and Final Final Boss will be damaged.

Note that his tail is the only way to hurt him, the rest is just to stun him properly. Keep up the tail a lot, and he will eventually die, and you will be rewarded with the Perfect Ending Cinema Scene.