OK! Then my very first thought: see if you can get hold of foam for most of this. I think that it would probably be the best way to make the truly 'grotesque' shaping, by being able to melt and shape it a little more easily. Price out those panels for judo floor mats or kids' play-pieces. It's thicker than 'fun foam', but I think you'll need the thickness to shape the head.
It won't look as realistic, but you probably want to make the vertical 'mouth' in the middle middle a little more open, and wear a black see-through hood so that you can breathe properly. That'd be a lot of foam to have around your mouth; you'd probably stifle pretty badly.
For the body, strips of cloth and/or cardboard should make the various parts of the body suit no problem, and the sticking-out bits of metal can probably be fabbed up with fun foam or glue-stiffened cloth. I'd avoid using real metal on it as it would make the stuit that much heavier, and that much more dangerous (and most cons don't like pointy stuff if it can be helped).
The sword would actually be pretty easy: take a cardboard or foam mock-up of the sword, put it inside a sleeve for your arm, put a handle at the end. Then you can just wear the weaopn as a 'sleeve'. Don't tape it into place, though; it might not look as realistic, but if an emergency happens, you want to be able to get your hand free fast.
http://www.cosplay.com/costume/428004/
You can ask more detailed questions about this guy who did one in Finland, too. He can probably give some ideas about how he did it. His head doesn't do what I suggested, but I'll wager that it was nothing at all like 'breathable' because of that. And I like cosplays you can wear for at least a few hours without having to pull off the head to get oxygen.
Help Needed for a Costume!
