One of the requirements to fill the Mission Gauge of the "Kill At Least Six Giant Heartless" mission is to defeat Dustflier. This is the strongest Boss of the game beside Ruler Of Sky, Saix, Riku and Final Xion Form. Make sure to lock-on this Boss in combat and run quickly from it when it started to fly and stomp the ground. This move can kill Roxas with one hit, and cause status ailments or bad status. It has a large amount of HP and is immune to any magic -- be patient. Attack and run as you can, and make sure you can dodge all of the Boss' attacks. Equip the Zero Gear with two Ability Units, two Power Units, and one Guard or Magic Unit. Bring fifteen Megalixirs, five Cures, five Curas and five Curagas. If your level and stats are high enough, you should eventually win the battle.