You are seriously
fighting physics on this one. There doesn't appear to be any mechanism to keep the pauldrons from tipping off your shoulders. I can only see a couple things that might work.
You can adhere the things onto her shoulders. You'd have to build the pauldrons so that the underside conforms to the shape of the shoulder so you can have the needed surface area to create a secure bond. You'd need to make them light, so like, craft foam. And you couldn't really remove them and reapply them repeatedly during the day, because it is bad for the skin.
You could have a rigid connection between the pauldron near the armpit and the purple underbust section. Based on the side shot, they don't appear to be connected in the character design, but oh well. The rigidity would be critical, otherwise the connection would just twist and the pauldrons would tip. You can either make the entire purple section of both the pauldrons and the underbust rigid, or you could use something like a 3/16'' steel rod that runs along the inside edge of each pauldron and the top of the underbust; tucked underneath.
If doing the purple parts rigid, I'd probably go with fiberglass on this. There's a chance that sufficiently thick Sintra would be rigid enough, but I am not familiar enough with it. I'm almost certain that wonderflex is too flexible to work here. To do the fiberglassing, you can sculpt the form in soft clay, paint the clay with cheap acrylic paint or Krylon Crystal Clear spray enamel, then give that a couple coats of PVA mold release, then fiberglass away. 3 layers of fiberglass cloth is good (don't use the chopped strand mat for this application) and squeedgie/scrap off as much resin as possible before it starts to gel up. Once it begins to harden apply either a gel coat or a thickened coat of resin to hide the fabric texture. Once that it set, you can peel everything off the clay, sand it smooth, and paint it.
If you make the purple parts rigid as described, you could potentially have them as separate pieces if you can fasten them together securely. This wouldn't look quite as good, since there'd be a visible seam-line, but it would facilitate things like bathroom breaks. A couple square inches of industrial Velcro would probably work, or maybe a couple screw posts
There's a third option I'm not so sure about. You could embed lead weights to the lower inside corners of each pauldron. If you can add enough weight, and the black part of the pauldrons is light enough, you might be able to balance the pauldrons and lower their center of gravity sufficiently that they just hang in place. I fear that it would require so much weight that you couldn't manage to embed it all, but I'm not about to start doing the torsional force equations to figure it out.
Regardless of the method, I'd recommend doing both the black and red parts of the pauldrons in craft foam. The flexibility would increase your range of motion and be safer too.
Hope this helps