Anti-Saïx

The Anti-Saïx (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 as a boss at The World That Never Was in Nothing's Call.

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 enemy 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, or you will be hit with a devastating counter attack.

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.

Trivia

 * It is unknown if this enemy is a Heartless or a Nobody. It has the appearance of a Heartless, yet uses the form of a Nobody.
 * It is possible that Anti-Saix is Saix's heartless counterpart.