This quest comes from the Quarry Foreman in Cut-Cliffs, on the road to Meridian. After the initial conversation with him, head down to the mining area. Investigate the mound or the explosives, but it honestly doesn’t matter as after a minute or so, a Rockbreaker will break the surface of the ground and the mystery of what happened in the mind becomes clear. Your task is to kill the Rockbreaker, which is not easy. Its got a lot of health and its ability to dig underground and pop up and hit you hard is a difficult one to combat. Also, don’t bother climbing up on top of the rock piles in an attempt to escape it, it will just leap up and land on you.
Your best bet is to keep moving, waiting for it to make a leap for you, then hit its digging arms with high Tear weapons. Once you have the arms disabled, this machine becomes a lot easier to fight as its primary weapon is gone. When it can’t dig, it will either try to lunge at you if you’re close, or it will fire boulders at you if you’re at range. These attacks still to a lot of damage, so be aware. You can utilise blast wires from your tripcaster here to good effect, and if you get a chance either hit the blaze on its underside or its exhaust ports. The trick to this fight is to make sure you’re reading its movements and dodging well, because each hit will hurt. You can also use the Blaze canisters scattered around the area to do some more damage by hitting them with fire arrows when it’s nearby.
All of this should combine to you taking it out. This will probably be a tough fight, but it’s worth it just for the loot you get off the Machine, let alone the quest reward. When it’s down, talk to the Foreman to get your reward.