This is a worthy movie as your last flash!
The reason why zero lost, is actually because Plantette evolved into Plantette Prime, which by the war far exceeds Zero in power and summoning capabilities. Thats's why Zero started to look injured after Plantette Prime's first attack. Now if Zero had evolved into Megaman Zero, which by the way is actually the fusion of Zero and Megaman X. Looks like 32 bit zero but with long yellow hair and red battle armor. Now that could slice and dice Plantette Prime in no time at all. However since, Headdy is a moron, and Ristar is the man, will in comparison to Headdy. That is the true reason why Ristar won, because Headdy being a moron, didn't bother to locate Zero evolution material, unlike Ristar who happen to be smart enough to find his for plantette.