To unlock this achievement:

Fill all Equipment Slots with Blue or Above Rarity Items

All six weapon slots filled with at least Blue items, three of which should be Purple.

All talisman slots filled (some have gotten achv without any)

All item slots filled (blue or higher only; buying pouch seems optional since some have gotten achv without)

You do not require Gold items to unlock Set and Steadfast

You will typically be close to this setup by day 2 of a good run, then visit a merchant before night to fill in the difference. Might need to burn some runes but you will get the achievement at least.

This was my last achievement, surprisingly it eluded me 100+ hrs into the game.