Anti-Saïx

The AntiSaïx (アンチサイクス) is a slightly weaker dark version of Saïx that is found in Kingdom Hearts: 358/2 Days. It appears as a secret boss in Mission 91, replacing Saïx at The World That Never Was.

Strategy
Anti-Saïx uses the same tactics as the true Saïx while in battle. It will immediately power up and go berserk at the start. Don't bother to use Lock-On, as this dark Nobody immediately begins the assault. The boss will continue to hammer the player with its dark version of the Lunatic, so it is important to heal regularly and also make use of the Limit Break. Only attack once the boss has settled down.

So long as the player follows this strategy, Anti-Saïx shouldn't be much of a challenge. Keep in mind that since Anti-Saïx is only fought in Mission Mode, the characters the player chooses to play as will have various effects on this boss battle.