After defeating the bird of Pirta, you will be head8ng back to The White City. Once there, instead of going to rest. Head towards the green marker to explore the Grand Core. This is quite a long section, with several traps and puzzles to overcome. Which in all honesty, are that troublesome. Once at the end, you will see a green chest which is not accessible until you complete the final puzzle. Once you've done this, the chest will become available, and you can then unlock the shield, along with this achievement.
Do not think this is story related, as its not a main mission. Nor am I sure if you can come back later to do it. Most likely you can, but worth getting out of the way.