Dark Bahamut is a boss in Final Fantasy IV. It guards the Ragnarok, Cecil Harvey's strongest weapon in the original release of the game. Dark Bahamut begins the battle with Megaflare, and then casts Reflect on itself and bounces Flare spells onto the party. However, its power is still considerably below Bahamut's - Flare is typically not fatal, and it is possible to survive Megaflare as well. It is important to note that Dark Bahamut counters summoning with another Megaflare, severely limiting Rydia's potential in the fight - although she can still summon Bahamut to pierce Reflect, this will cause Dark Bahamut to counter with Megaflare.
For the SNES and the GBA releases, after Dark Bahamut uses its first Megaflare, revive all KOed characters with Phoenix Downs while Rosa casts Curaja. Continue to cast Curaja with Rosa while Rydia summons Asura. This way the entire party stays alive easily and benefits from the Protect status. Cecil should attack physically with his sword, Kain should utilize the Jump command and Edge should ThrowShurikens at the boss.
Another strategy here is to immediately use a Spider's Silk or have Rosa cast Slow on Dark Bahamut. Now, if you decided to give Rosa the Omnicasting Augment, you can simply have her cast Blink on the entire party in just one turn, followed by Reflect on the entire party. Then you can feel free to use physical and magic attacks without the consequences of Dark Bahamut's counter attacks including Flare. If he casts Reflect on himself, just cast Flare onto yourself if you have Reflect on your party then it will reflect onto him. Although, when he uses Flare as a normal attack, he will bounce it off of his Reflect barrier and onto you. But as a counter attack, he will cast it directly towards you, bouncing it back to himself. Be sure to recast Blink if it wears off. Also with the Omnicasting Augment, if multiple members of your party get KOed from the opening Mega Flare but Rosa survives, you can cast Arise on the entire party and it will fully revive everyone.