Chances are, you've never heard of this beast, let alone fought it. The battle is optional, so you're not required to slay the wyrm. The requirements to fighting it are ridiculous, and then once you figure out where to find the beast, he'll probably wipe your party out in 20 seconds. Let me throw some stats out there to give you an example of what we're dealing with: over 50 million HP (Emerald Weapon of FFVII had 1million HP), capable of inflicting Instant Death, and once you take him down to 50% health, his damage cap is 7,000 HP.
Now the positive is that you can leave the battle anytime and save the game at anytime. The negative is that he'll cast Regen on himself before you leave and be healing himself even when you're not there. His best trick though is that he'll cast Reflectaga on your party, and if you cast Renewon yourself, it reflects to him and he gets all of his health back (tricky bastard).
He's the strongest Final Fantasy boss in the history of the franchise, and if you were one of the few who were able to defeat him, the you get my seal of approval as a true gamer.