Berder's Octopode Fighter of Dithmenos Guide
I'm not going to lie: octopodes are a challenge. Are you up to it? With their near total lack of AC, octopodes might seem fragile, and they do have to be very careful about not getting surrounded by more than they can handle. If you just want to win Octopode, I'd recommend something like OpFE where you can blast away from a distance without getting your tentacles dirty. But with care, an OpFi can crush an impressive number of skulls, and their stealth often lets them wander through a carnival of death completely unscathed.
Here's my winning OpFi, as an example of what to look forward to: http://dobrazupa.org/morgue/Berder/morgue-Berder-20141201-063046.txt
And here's my other OpFi, who died to an ancient lich's chain lightning, but not before getting 9 runes and destroying Cerebov in melee: http://dobrazupa.org/morgue/Berder/morgue-Berder-20141129-170200.txt
You are going to be pumping Dex because you need more dodging, so I would recommend using long blades. Long blades cross-train with short blades. This means you can keep a dagger in your inventory in addition to your long blade main weapon, and stab unaware enemies with the dagger, assuming you bothered to put any points into the Stealth skill. You don't need to, but it's an option.
I recommend Dithmenos. The first thing Dithmenos does for you is reduce the accuracy of enemies within your umbra. This helps you immediately, as soon as you get an umbra. Your defenses are EV and SH, both of which are more effective when the enemy's accuracy is lower, so Dithemenos is especially good for octopodes.
The second thing Dithmenos does for you is drastically increase your stealth. With Dithmenos, you don't actually need to train stealth to be stealthy. An octopode at max Dithmenos piety with 2 rings of stealth, in water, is "uncannily stealthy" - even at 0 stealth skill. This is quite useful because it lets you pick your battles. If you encounter a crowd of enemies, odds are none of them noticed you at first, so you can throw a rock at one of them to lure him away while leaving the crowd behind. Stealth is also quite handy in the abyss because all those dangerous enemies just don't see you.
The third thing Dithemnos does for you is shadow step. Unless you choose to get some Stealth skill and do stabbing, shadow step is mainly useful in lair, because you can step to plants. That's a handy way to escape a death yak or hydra that got too close for comfort, without using up any valuable consumables.
You aren't that weak because you have constriction, but be super careful. Lure enemies one at a time to your position. Mark exclusions on anything you aren't sure you can kill (assuming you didn't wake it up). Skip the level if dangerous fast or ranged enemies are wandering around in it. Don't fight dangerous things like ice beasts or ogres without an escape plan if things go south. If you find a blowgun, use it as much as possible to soften up dangerous enemies.
First, train only Fighting and Long Blades equally, to 14 skill, which is minimum delay for a long sword. You'll achieve this by early lair. You may also want to concurrently train some shields.
For now, you can't kill hydras, so run away or use shadow step to escape them, and mark exclusions if they are sleeping (and try not to wake them).
Next step is to get some more Shields and Dodging skill. Start training both. Train Shields a bit more than Dodging, because your shield is slowing your attacks a bit as long as your skill is beneath 15. But you don't have to train it to 15 immediately. Just get like 8-10 and you'll see a big improvement in your defenses.
Eventually, after you've gotten some more shields and dodging, you might be fed up with hydras. That means it's time to get either evocations or throwing. Get evocations if you've found a good rod for killing hydras. Get throwing, otherwise. Train whichever skill you get to about 10, which is sufficient. You might want to do Orc around this time to get more ammo if you do throwing; doing Orc after you've done part of lair is a good idea, anyway. Now you can go back and kill those hydras you left alive.
The next step is to get 15 shields skill, to completely eliminate the penalty of wearing a shield. Then for the rest of the game, train mostly dodging, fighting, and long blades, plus throwing (if you're using throwing) or evocations (if you're using evocations).
If you want to, you can train some stealth and then you'll be able to stab things like I mentioned above. Totally optional, but your stealth aptitude is very high at +5 so it's not too big an investment to get like 10-15 stealth.
You can also get like 10 invocations to eliminate the failure rate of shadow step and let you use shadow form. Personally, I've never needed to use shadow form, but it could potentially be handy in an emergency.
Even if you don't get evocations at first, you should get some eventually so you can at least evoke teleportation.
Late in the game, maybe by depths, you can get Statue Form. This will improve your durability a lot. If I were you, I wouldn't take on Vaults:5 without it.
Bolt of Inaccuracy
If you get a Rod of Inaccuracy, use it! In addition to its normal uses, you can switch to it in melee after constricting a dangerous enemy. Constricted enemies have their EV dramatically reduced, so unless they had super-high EV beforehand, they become viable targets for Bolt of Inaccuracy.
Invisible Enemies and Unseen Horrors
These are more dangerous to octopodes than they are to other characters. Enemies that you can't see bypass most of your EV and SH, so you can't last very long against them. If you're getting hit by an unseen horror you read tele or go up stairs immediately; don't use the corridor trick to try to kill them, because it won't work. Also, be cautious about invisible orc wizards. Escape and come back when they aren't invisible, if possible.
After the lair runes, the easiest rune for you is probably the abyssal rune. Your stealth makes the abyss quite safe for you. Just keep your ring of teleportation on, walk away from monsters you don't want to fight, and teleport if you see danger.
Amulet of Guardian Spirit
This is a good amulet to keep on since your HP is on the low side. It really improves your survivability.
Some enemies cast paralysis. Great orbs of eyes, ogre mages, orc sorcerers, sphinxes, liches and ancient liches. This is absurdly dangerous to you because when paralyzed you lose your EV and SH and don't even have any AC to soak up hits. A band of ogres with an ogre mage in it could be a mortal threat at XL 20, even though you could usually turn them into mincemeat. If an enemy can paralyze you, try to fight it one-on-one or else escape/teleport out. Having a ring of MR or two helps, and you might want to switch to an amulet of stasis if you have it and one of these enemies comes into view.
Nikola and ancient liches can cast chain lightning, which you can't dodge or block, and which can deal vast damage (think 100 damage per turn) if you have no rElec. Do not fight these things without rElec and maximal caution. Ancient liches are a double threat due to the possible paralysis.