Old Iron King

This boss is easy for ranged characters, especially miracle users. There's a safe spot in the side of the building where the boss' fire balls can't reach. Beware of his fire beam as it can penetrate the wall and kill you while you're still recovering back to your feet. All of the boss' attacks can knock you back to the lava, resulting in instant death. Sorcerers who rely on pyromancy will also have to adjust since the boss is obviously resistant against fire attacks.

When you see the boss dive into the lava, quickly change position and run to the other side since he will appear on the same spot where you're standing a few seconds after. When done correctly, he will resurface in the far end of the stage, allowing you to snipe him and protect yourself from his slow but powerful fire projectiles.

Melee characters need to get close to the boss to prompt it to use melee attacks. It will usually be vulnerable after it slams its fist to the ground so use this chance to get up close and land heavy damage to the boss. If the boss uses its fire projectiles and beam, just stay behind cover or evade. If it dives, lure it to the middle of the battleground; that way, you can easily run for cover and attack. Just repeat this pattern to ensure victory.

One does not simply fight the Balrog

After defeating the boss, enter the ruined building and loot the chest to get a Sublime Bone Dust. Go downstairs and light up the primal bonfire. You can also use it to return to Majula.