1. Unobtanium
<PERSON>'s hobbits wore mithril, a super light and super hard metal. The Lokk might have access to large quantities of such a material and could armor an army with it.
1. Super Advanced Old Technology
The first carbon fiber was produced in the mid-1800's. The greatest Lokk inventor perhaps figured out how to spin coal ash into a hard but brittle thread, and applied a resin coating to form the substance into a plate. The result is a hard and lightweight but brittle material that can take a punch or two before it shatters. You might add a thick fabric behind it to provide a little protection against piercing weapons.
The difficulty in lots of modern technology isn't necessarily the fundamental idea, but rather the prerequisite scientific understanding or the development needed to make something practical at a large scale. Neither things most readers care about. Lots of similar technologies exist.
1. Super Advanced Old Technology 2
Large quantities of Aluminium were not available until after the invention of the Bayer Process in the late 1800's. The Bayer Process itself was discovered by accident. 561
The Lokk could have had that accident much earlier, or they might have access to native (pure) aluminum deposits.
Aluminum is a relatively strong metal (although we usually use it in thin sheets, so it might not seem like it), but not quite as strong as steel. By density, aluminum is about 3 times lighter than steel. According to various rules of thumb I found online:
* If a medieval set of steel plate armor weighed about 110 pounds, then an aluminum version would be about 75% as strong but only weigh 36 pounds.
* An equally strong set of aluminum plate armor would be a little bulkier, and weigh about 55 pounds
* Aluminum is more malleable than steel. This would make it less effective versus blunt weapons and somewhat more less effective versus piercing weapons, since the metal deforms easier. On the flip side, the material is easier to craft with and repair, and can be formed into more complex shapes.
It is relatively abundant as well.
1. Super Advanced Old Technology 3
Titanium in this context would be similar to the above discussion of aluminum. It became viable to produce in bulk in the early 1900's through a directed research program in chemistry. Titanium is about half as dense as steel, but is about as strong. If a medieval suit of plate armor weighed about 110 pounds, then the titanium version would weigh about 55 pounds but be dimensionally similar (unlike aluminum, which would be bulkier).
One big caveat about titanium is that it would be significantly harder to work with. Not that your medieval Lokk would be welding, but titanium welding must be done in an inert atmosphere (argon or nitrogen) or else chemical corrosion will embrittle the material. It can actually be shaped similar to stainless steel, but readers might assume that titanium simply can't be worked by hand. 561