The Ocotopus boss is really quite simple.
His first assault consists of three kinds of attacks:
1) Tentacle crush - A tentacle will rise above your character and follow you for about 4-5 seconds before coming down dealing a good chunk of damage. Avoid this by running in one direction. It should trail just slightly behind you, but sometimes it'll stay right on top of you.
2) Tentacle whirl - Octopus twirls it's tentacles about very fast dealing an average amount of damage per hit, although it will hit you about 3-4 times with the last one dealing critical damage (as a numan, it may only hit you about 2-3 times due to your higher evasion rate). You should keep your health at about 2/3rds full if you expect to survive. If it kills you with 3/4ths health, go level some more. I don't know how to predict this, but I believe if you can find out, going to the outer edge of the battle ground should put you out of range.
3) Suck me, beautiful - Octopus creates a vacuum with it's mouth drawing vast amounts of pressure inward. While your characters won't be eaten, it does give it a good chance to hit you with it's spitting attack which will hit you about 3-4 times dealing slightly above damage with each hit. Odds are, if you're not at full health, it will kill you. You can predict this by watching it approach the opposite end of it's pool. You will have the time it's drawing air into itself plus about 2-3 seconds to get as close to it as possible (go around the pool and approach it's exposed head, you can get a few extra hits in this way as well.
Round two, FIGHT!
1) The octopus uses it's remaining 4 tentacles in this battle, with two of them being the forefront tentacles (I don't know the name, but I know they have a different name when referring to actual octopus') which can deal harsh amounts of damage. The good thing about this battle is that if you can survive it's critical hits, as long as you have a good stock of monomates/resta w/ monofluids, you shouldn't have too much trouble. Just keep bashing in that octopus face and you will eventually win. Keep your health high.
(if you're still having trouble, let me know. This is the hardest boss, for me, next to... err... nevermind.
