Instructions
Back away from Eraqus as soon as the fight starts, and start circling around him, keeping him at medium distance from your character. Eraqus has several attacks that he can perform with no warning and that do significant damage. You want to make sure you keep constantly in motion to avoid them.
Press the "Square" (Block) button to counter Eraqus' attacks when he comes at you with either his multi-hit Keyblade combo or his triple Keyblade charges. This will knock the boss off balance and allow you to get in a Keyblade combo of your own.
Block Eraqus' ranged attacks as well. Make sure you time the block at the point just before his attacks hit to reflect them Legend-of-Zelda-style back at Eraqus. This will cause major damage to the boss when done properly.
Repeat these steps as needed until Eraqus changes his patterns. After you damage him about one and a half life-bar's worth, he will add three new "desperation" attacks to his repertoire.
Keep moving around. Do not stand still if you can help it. Eraqus now has access to a "teleport" move that makes him appear right next to your character, at which point he will hit you twice with almost no warning. If you keep moving, you have a chance to avoid this attack when it comes.
Prepare to dodge and slide when you hear Eraqus yell "Prepare Yourself." He will leap into the air and summon pillars of flame all around the battle area. These flames can be dodged by sliding through them. Try to get next to Eraqus as he comes down after this attack and get in a damaging combo while he is vulnerable. This is not always safe, but it is a chance to do some damage.
Retreat from Eraqus, and then stand perfectly still when you hear him yell, "Let the Darkness Die." He will summon a ring of laser bars to surround you. The ring will then start rotating, and you will have to dodge through the bars as they move. Try to hit Eraqus with a fast move -- such as Sonic Blade -- after this attack. Be wary, however, as Eraqus often uses the teleport-strike immediately after the laser cage.
Cast spells and used ranged attacks as much as possible on Eraqus during this fight, especially spells such as Fireaga or Iceaga. These spells will not do much damage, but will chip away at Eraqus' life gauge and will also fill up your Command meter, allowing you to do a Finishing move more often. Finishing moves should be used as often as possible.
Continue attacking and dodging Eraqus' attacks -- as well as countering the attacks described in steps two and three -- until Eraqus is defeated. Once you kill the Keyblade Master, you will be treated to a cutscene -- at the end of which you will receive Eraqus' Keyblade "Chaos Ripper."