This is a story achievement and unmissable.
When you go to The Quarry for the first time and try to progress farther your weapon will be damaged after a sudden cave in, return to the town and to the Outsider's Lane where you'll meet up with the man you helped at the start of the game, after a brief chat he'll introduce you to the blacksmith who asks you to mine some iron ore from the Quarry, he gives you a pick axe to be able to do this.
Return to the Quarry and mine some iron ore from the deposits, bear in mind that the monsters will be starting to getting stronger now. Once you have five pieces return to the blacksmith and the shop will be built and opened, Garoo, who looks like a kangaroo of sorts and has a rather large sword, will arrive to have his sword sharpened.
After a chat, argument and some humour he'll be talked into teaming up with you with the achievement popping not long after. Garoo's special ability is being able to smash the larger non-glowing rocks that you may have already encountered in the Quarry and in the Great Forest, this can be helpful for farther progression.