Octopodes? No way were they built for transmuters. Monks all the way, bro.
How aren't they built for transmuting?
An octopode in a form gets to keep his eight rings, whereas most equipment on most characters gets melded, so an octopode suffers less penalty from changing from relative to other characters. Plus you have wacky tentacle auxiliary attacks!
Max hit the ball on the head. Get 8 rings of mana and protector spirit, and you're set.
This is a bad idea. You'll effectively get +9 max MP from each ring... yet you'll be missing out on every single potential effect you could have on a ring or amulet, which amounts to a rather poor character. I mean, come on, you wouldn't swap one for wizardry, or slaying, or a nice randart?
Also, all bonuses that would bring your MP above 50 get halved. If your natural max MP is 20, 8 rings of magical power brings you up to 71, not 92. If your natural max MP is 35, 8 of them brings you up to 78.5, not 107.
That's on top of the fact that you simply aren't going to find eight of those in one game.
Alternatively, get 8 rings of slaying, and OHK anything that comes in your LoS.
Yeah, except for the fact that you have to get to them first. You're still going to get hit by things even if you're capable of killing them quickly... especially if you're giving up most of your defense for it.
And as above, you're not going to find 8 rings of slaying in a single game, much less good ones.