Which is hardest: iron, brass or bone?
These two sources both put bone at a hardness of 5:

http://www.chacha.com/question/how-hard-is-bone-according-to-moh's-hardness-scale

https://answers.yahoo.com/question/index?qid=20110310200841AABwtMj

Whether they are trustworthy is questionable though, so take it as you will.

This source put brass at 3 and iron at 4.5:

http://www.jewelrynotes.com/the-mohs-scale-of-hardness-for-metals-why-it-is-important/

and this image puts brass at 4 and iron at 4-5 (Similar to 4.5):
http://patentimages.storage.googleapis.com/WO2001048807A1/imgf000009_0001.png

While these different sources seem to have conflicting data, I think it would be safe to assume that Brass is the softest of these three materials, Iron comes second, and Bone is the hardest.

Edit: In the description of that monster, the adjective used is 'strong'. You may want to consider how much force each of these materials can withstand instead of how hard they each are :)