How to train and farm guardian?

Each guardian in Guardian Hunter has its own role. Some can be used in battle, some can be sold for gold, and some can be used as material (food) to make your primary guardians become tougher and stronger. But the question is: How can we know the role of every guardian?

Every guardian has its own grade (star) and by only checking it you will be able to know which guardian you should train and which guardian can be sold or used as a material (food).

My personal classification of guardians:

Every guardian that are grade (star) 6 are considered as fighter guardians. However, grade (star) 6 duplicate guardians can also be a material (food) guardian because it can enhance your guardian and more importantly they can be used to awaken your guardians.

Every guardian that are grade (star) 5 and below are classified as material (food) guardians. However, and for me, grade (star) 2 and below guardians are just for selling purpose only.

More importantly, you should know where to farm your needed guardians. You can simultaneously hunt your material (food) guardian and fighting guardian to save you more time and effort. Some examples are Stage 20 Elite for Sandworm, Stages 37 or 38 for Lycan Warrior, Stages 39 and 40 for Barlog and so on...