This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Bahamut will slowly count from 5 to 1, and when time runs out, he'll
cast MegaFlare on the party. The force of the spell will wipe out your
entire party in one turn...
To defeat Bahamut, you need to use the same technique you used to defeat
Asura, except this time you should put those Reflect spells on your
party. This will reflect the power of the MegaFlare attack back on
Bahamut and save your skin.
Wyvern is exactly the same as Bahamut, except he doesn't do a countdown
before he launches a MegaFlare attack - he attacks immediately.
(Fortunately, Bahamut's MegaFlare is stronger than Wyvern's.)