You'll find two nice items on this floor, the Hades Armor, and a Hermes Item that grants Haste to its user. This is a valuable effect against bosses, and whether you want to use it on Octomamm or wait until a tougher boss is your call.
Octomamm will attack when you move up to it in the Waterfall. Cecil's sword is highly effective, as is Tellah's Bolt1 Spell. Rydia will get more mileage out of summoning Chocb. Feel free to use any consumable Items you might have picked up as well, if you want. Like most bosses, Octomamm is immune to Status Ailments, so don't bother with Spells like Hold or Venom.
Octomamm will attack when you move up to it in the Waterfall. Cecil's sword is highly effective, as is Tellah's Bolt1 Spell. Rydia will get more mileage out of summoning Chocb. Feel free to use any consumable Items you might have picked up as well, if you want. Like most bosses, Octomamm is immune to Status Ailments, so don't bother with Spells like Hold or Venom.