One of four 'advanced' Jobs found in Octopath Traveler II, Armsmaster is quite similar to the Warmaster from Octopath Traveler. The bearer of this Job can equip all six weapon types, and the Skills that come with each weapon are quite powerful. Just unlocking the Job is not enough to make it truly powerful, however, nor will JP alone get you all six Skills. You'll need to do some exploring to make the most of the Armsmaster Job.
Unlocking Armsmaster
In order to unlock the Armsmaster Job you need to travel to a town called Gravell. It's located far in the north of the Wildlands, on the western continent. To get to Gravell you'll need to follow the northbound path from Crackridge, from the Western Crackridge Wilds to the Western Gravell Wilds. Gravell is on the east side of the Western Gravell Wilds. The enemies in these parts average at level 45, so the journey won't be easy for low-level parties.
Next to Gravell's save point is a small building with a Debt Collector out front, blocking your way in. You'll need to defeat or otherwise knock out this man to get inside the building, which turns out to be the home of a blacksmith. The blacksmith, who goes by the name Porta, is retired... though he'll tell you about six divine weapons of legend. If you happen to have any 'Rusty' weapons in your inventory Porta will recognize it as one of the divine weapons, and he'll give you the Armsmaster License, allowing you to change one of your party members into an Armsmaster.
The Armsmaster Job begins with three Skills: Sixfold Strike, Bulwark, and whichever Skill was attached to the Rusty weapon that you showed to Porta. To learn the rest of the Skills you'll need to track down their associated weapons and bring them to Porta, who will then reforge the weapon into its original state. This will begin the In Search of the Divine Weapons Side Story, a quest that will send you all over the world looking for Rusty weapons.
The Skills gained by bringing Rusty weapons to Porta are unlocked for everyone in your party, without the need to expend JP. That said, your Armsmaster needs to equip the weapon associated with each Skill to use the Skill. If you want to use Cosmic Roar, for example, your Armsmaster will need to be holding the Conqueror's Sword. (If it helps, the divine weapons are all pretty good end-game arms.)
Rusty Sword
The Rusty Sword is located in Winterbloom, in the north of the Winterlands. You'll find this town to the east of Cape Cold, where Osvald begins his journey. Check the east side of town and you'll find a woman trying to remove an old sword from the ground. Speaking to her will begin The Sword in the Stone, a Side Story.
The woman needs something to dig the sword out of the ground. You'll find that something if you visit Oresrush, Partitio's starting location. Pay a visit to the Armorer, in the northeast of the town, and you'll find a Retired Miner. He has a Sturdy Pickaxe in his inventory that you can Steal, Entreat, Purchase, or Mug. Take this back to the woman in Winterbloom and she'll dig out the Rusty Sword for you.
When reforged the Rusty Sword becomes the Conqueror's Sword, and unlocks the Conqueror's Sword: Cosmic Roar Skill for the Armsmaster Job.
Rusty Polearm
The Rusty Polearm is located in Timberain, a city in the northwest of the Leaflands. To reach Timberain you'll need to travel west from Cropdale, from the Southern Cropdale Trail to the Southern Timberain Trail. Timberain is in the north of this route, so long as you travel along the southern trails and make your way west.
Head north to Timberain Castle: Town Square. On the west side of this area you'll find an Elderly Soldier guarding a door. He will challenge you to a duel, and remains in this spot all day long. If you're feeling sneaky you can use Ambush on the guy, though otherwise you'll need to fight him one-on-one via Path Actions. He's a powerful combatant, so don't take him lightly. Beyond the Elderly Soldier you'll find a storeroom, and if you check the right side of the storeroom you'll find a loooooong path to the chest containing the Rusty Polearm.
When reforged the Rusty Polearm becomes the Warlord's Spear, and unlocks the Warlord's Spear: Giant Swing Skill for the Armsmaster Job.
Rusty Dagger
The Rusty Dagger is located in the Shipwreck of the Empress, the remains of a sailing vessel stranded in the middle of the Sundering Sea. To reach the dungeon you'll need to complete Partitio's Scent of Commerce quest in Tropu'hopu, which earns the party a ship of their own. You'll need to spend 100,000 leaves to buy the boat.
Once you've navigated the seas and made your way to the Shipwreck of the Empress - it's north of Canalbrine - you'll find a Danger Level 48 dungeon waiting. The chest containing the Rusty Dagger is located in the captain's quarters, on the west end of the ship's upper level.
When reforged the Rusty Dagger becomes the Dancer's Blade, and unlocks the Dancer's Blade: Mischievous Step Skill for the Armsmaster Job.
Rusty Axe
The Rusty Axe is located in the Quicksand Gaol, a Danger Level 30 dungeon located in Hinoeuma. You'll find the gaol by traveling west from Sai, to the Western Sai Sands. The Quicksand Gaol is in the northwest of the route. The chest containing the Rusty Axe is right in the middle of the dungeon, at the bottom of a sandy pit, and you'll need to defeat the Lord of the Sands to open the chest.
When reforged the Rusty Axe will become the Lionheart's Axe, and unlocks the Lionheart's Axe: On the Hunt Skill for the Armsmaster Job.
Rusty Bow
The Rusty Bow is located in the Sinking Ruins, a Danger Level 46 dungeon. You'll find the dungeon on the west side of the Southern Nameless Village Traverse, a route to the north of the Western Trop'hopu Traverse on the islands of Toto'haha. You'll need to defeat the Behemoth, the dungeon's boss, to get at the chest containing the Rusty Bow.
When reforged the Rusty Bow becomes the Hunter's Bow, and unlocks the Hunter's Bow: Flurry of Arrows Skill for the Armsmaster Job.
Rusty Staff
The Rusty Staff is located in the Seat of the Water Sprite, a Danger Level 26 dungeon. You'll find it in Borderfall, a route in the Crestlands that connects Flamechurch and Montwise. The dungeon is found at the bottom of a waterfall, accessible by climbing ladders on the east side of the route and then sailing out in your canoes. In the north of the dungeon you'll find a ladder with waterfalls on either side, and the chest containing the Rusty Staff is at the bottom of the ladder.
When reforged the Rusty Staff becomes the Spiritlord's Staff, and unlocks the Spiritlord's Staff: Deliverence Skill for the Armsmaster Job.
Finishing Up
Once you've collected all six divine weapons and reforged them, Porta will congratulate you on your efforts. You'll then complete the In Search of the Divine Weapons Side Story, receiving 20,000 leaves and a Wise King's Shield in the process. Well done!