I'm not too much of an expert in sewing, so excuse me if I have no clue what I'm talking about when it comes to that, but I absolutely love Nu-13/Lambda-11 from the series and I think i can toss a few ideas around.
If you're only doing the base outfit [the leotard, shroud, eye patch + wig]. And you're looking to modify clothing. My opinion would be to take exercise clothes. Yoga capri pants and a tank top, sewing those together but then modifying it so theres a zipper on the back. Essentially making a onesie for her to fit in. There are definetely dyes for the powder blue color for her suit, and for the extra triangle on the outfit, craft foam would work. I was going to suggest normal styrofoam, but that around the crotch region would suck... For the shroud, I've actually done this because it was fun to swing around in. What you can do is find some cloth reminants that are white and powder blue. With the powder blu just cut the design, and with the white cut the overall shape of the shroud, then you caneither glue or sew that together to make the whole effect. For the papers in the back, i just took normal sheets of copy paper, folded them over a few times so they were sturdy and then painted the design on them. Then once that was done, get shower curtain rings [preferably get those black] and make sure there's holes in the shroud and the paper to take the ring through. For the front plate that is around her neck I'm not entirely sure how to do that. Its possible you can make a collar on the shroud and then use craft foam for the front plate to connect it together. Maybe velcro that so that it's removable if it gets too hot, or gets in the way.
If you want the large sword, get Density foam and sandwich it to the point of thickness you would like, or normal stryafoam [Density foam is heavy in a large quantity, but is thicker so you may only need two sheets of it. Stryafoam is more expensive and thinner., so even though you could mold it easier with cutting it down it would cost way more $$$ to make]. If you go the density foam route, take cardboard and place it on both sides of the weapon, this will make it so the foam won't bend as much and will look hardened. If you got strayfoam, make sure you take sometime to cover it before you paint it. The weapon is hard to make overall as a concept...may times i've tried and millions of times i have failed. I have seen it done....and it looks godlike if you can figure out what materials work well with each other.
Personally I am lost on if you want to do Lambda-11 form...again it's something i've seen done. And most of the time when i see it, it seems like people manipulate a lot of craft foam to the contours of the body. The mask is pretty simple to make, and so is the armor...but the legs...I have no idea how some people get it so large and able to stay up. I would say make a suit out of density foam, and then cover it with cardboard in areas with heavy edging [The spots that are to represent calves and thighs]. But as far as painting it or getting the colors to look metallic i'm stuck.
Either way I hope at least some of this helped out with some ideas to make the costume, I can't wait to see the progress photos for this as again, I love Nu/ Lambda and main them anytime i get a chance to play the game.
"These Immortals need to be taught a lesson."
Sun Wukong from SMITE
He Bo from SMITE
Xbalanque from SMITE
Possibly helping a friend being Nemesis from ...SMITE
Only one this year, Otakon 2014.