This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
This one's a little harder. Read the book about Odin in the Land of the
Summoned Monsters. Once you've read it, apply what you've read to when
Odin sticks his sword up into the air. (You may have to do this before
he does, or else he'll charge before you get the chance to pull it off.)
If you're really desperate, read on...
When it's Rydia's second turn during the battle, have her cast Lit-3
("Bolt3" in the PSX version). All other characters should attack as
normal. Keep attacking until the lightning bolt destroys Odin.
(Note: Only Lit-3 works. Lit-1 and Lit-2 barely faze Odin.)