Bravely Default Nemesis Strategy: Leviathan

Leviathan is a giant water creature whose lower portions are all you can see during the fight.  Being a water creature, it casts potent water magic.  This fight is far simpler than Belphegor.  This is for the level 30 version of Leviathan, higher level versions will obviously take longer.

Recommended party:

• Three Black Mages and one White Mage.
• The Black Mages should be at least Job Level 7, so they can cast the level 3 black magic spells, most notably Thundara.
• The White Mage should be at least Job Level 8, so they can take the Angelic Ward support ability.
• The Black Mages should also have White Mage at Job Level 8, so they can take the Angelic Ward support ability.
• As far as Job Commands go, as long as you have Miscellany on one of the Black Mages so that you can Examine, you're good.  Examine isn't strictly necessary, but knowing how much HP Leviathan has left is never a bad thing.
• For support abilities, you'll want Abate Water and Angelic Ward on everyone.
• Above level 30.  My party was level 35.
• Equipment that amplifies lightning, like the Lambent Hat, on all your Black Mages.

Special Moves:

• If you plan to use any, set the damage component to Lightning, and any elemental resistances conferred to Water.

Strategy:

• THUNDARA ALL THE THINGS
• heal as necessary
• ???
• Profit!

For higher level versions of Leviathan:

• Shoot at it until it dies.

Up next will be Tax on the Cupid.