Anti-Riku

The AntiRiku (アンチリク) is a slightly weaker dark version of Riku that is found in Kingdom Hearts: 358/2 Days. It appears as a secret boss in Mission 93, replacing Riku at The World That Never Was.

Strategy
Anti-Riku uses the same battle techniques as the true Riku. At the start of this battle, immediately Lock-On to the boss. This should aid you in keeping track of him, as Riku is fast. As soon as this is done, begin hammering Anti-Riku with the Keyblade or whatever weapon you wield.You will not be able to deal the finishing blows in some of your combos, as the boss will utilize Dark Shield and block them.A majority of this battle will be following this Anti boss around the battlefield in front of Memory's Skyscraper.

When you get close to Riku, he will attempt a series of melee attacks, a series of thrusts with the Oblivion he wields. He may also launch a Dark Firaga attack if the player is not within close range. When Anti-Riku's HP starts to deminish, he will begin to utilize his desperation attack, Dark Aura.Riku will jet around the battle area, unleashing columns of black flame at the combo's end. You can time this attack to when Riku's Oblivion shines with energy.

So long as the player follows this strategy, Anti-Riku shouldn't be a problem to defeat. Keep in mind that since this boss is only battled in Mission Mode, the characters you choose to play as will have various effects to this battle.

Trivia

 * Like the real Riku, AntiRiku is fought to the battle theme in "Another side, Another story...."