Dark Bahamut begins the battle with Megaflare, and then casts Reflect on itself and bounces Flare spells onto the party. Flare is typically not fatal, and it is possible to survive Megaflare as well. Dark Bahamut counters summoning with another Megaflare, limiting Rydia's potential in the fight, although she can still summon Bahamut to pierce Reflect.
For the SNES, GBA and PSP releases, after Dark Bahamut uses its first Megaflare, the player should revive all KOed characters with Phoenix Downs while Rosa casts Curaja. Rydia should summon Asura. This way the party stays alive and benefits from the Protect status. Rydia can also use the Quake spell to deal damage ignoring Dark Bahamut's Reflect. Cecil should attack, Kain should utilize the Jump command and Edge should throwShurikens.
Another strategy is to immediately use a Spider's Silk or have Rosa cast Slow on Dark Bahamut. If Rosa has the Omnicasting Augment one can simply have her cast Blink on the party in one turn, followed by Reflect on the entire party. If Dark Bahamut casts Reflect on himself, the party can cast Flare onto each other if the party is reflected to reflect it onto him. When Dark Bahamut uses Flare as a normal attack he will bounce it off his Reflect barrier but if Dark Bahamut uses it as a counter attack he will cast it directly, bouncing it back if the party has Reflect. With the Omnicasting Augment, if multiple party members get KOed one can cast Arise on the entire party to fully revive everyone.