I need to start with two frame challenges
First, you haven't said so, but I agree with @L.Dutch: your creature must eat organics like every other creature on Earth or it simply won't survive. The critter needs nutrition! But, as is famously quoted, your armadillo-sized-metal-eating critter, (let's call him ASMEC) cannot live on bread alone!
Second, as Gnudiff points out in his comment, armadillo-sized bugs aren't feasible. You can handwave this one, or convert the idea to an armadillo-sized animal. I'm approaching this somewhat from the animal side, but the answer could be read either way.
There is a glorious bacteria in the world, Cupriavidus metallidurans, that uses an enzyme to ingest toxic gold, convert it to a water-soluble condition, and then (*ahem*) poop it out as pure gold. That gives us a real-world basis for what you need.1
Introducing Freezium Pheonixus Metallidurans...
A stomach enzyme that breaks down (name your metal here or NYMH) into a water-soluble version of NYMH, which is then secreted like sweat such that it coats claws and (probably) scales to create (as it dries) an ultra-hard shell around ASMEC. You could even assume that the metal additionally serves the same function as grit does for birds: to help with digestion by grinding up the organics ASMEC eats.
The sweating process is slow enough that as ASMEC moves around, it creates the necessary articulation between scales, etc., to not become a solid statue. At a guess, you'd probably find this guy violently shivering from time to time to both help distribute the sweat before it dries and to shuffle everything to preserve articulation.
Have you ever seen a dog rid itself of unwanted water? Yeah... something like that. It would sound cool.
I noted your restriction and, though I've done my best, I'm ignoring it. Whomever is placing such a restriction on you knows too little about biology. As I mentioned in my (now deleted) comment, we don't have the tech to create life and don't have an example of an animal that does anything like this other than with hardened hair — so I consider the requirement of showing
that the system would work to be non-binding. Consider it a third frame challenge.
1 There is also a microbe, feroplasma acidarmanus, that eats iron. If you wanted an alloyed metal on the ASMEC, you could have the microbe live happily on its scales and let it bed down on iron ore. 😆 It's that much more to justify the believability of the ASMEC.