Farming it before you can clear Dragon Tombs efficiently. If you are struggling to beat the Iron Dragon, your time is better spent upgrading your starter weapon first. A +5 starter weapon clears faster than a fresh account trying to brute-force the drop.
Ignoring the fire damage bonus. The Fiery Iron Sword's main advantage is its fire damage. If you run it with a neutral build and no Fire Rune, you are getting a slightly-better stat stick instead of a weapon with real synergy. Commit to fire or switch weapons.
Over-investing in reinforcement early. The sword is a mid-game bridge weapon, not an endgame carry. Taking it past +8 is usually a waste of ore that could go toward your late-game weapon. Know when to stop and save.
Sleeping on the Poison Rune alternative. Fire Rune is the obvious choice, but Poison Rune gives you dual DOTs that melt boss health bars. Test both before committing. Some players prefer the Poison route for long boss fights.
Holding Cave Tickets for Dragon Tombs runs. Dragon Tombs is free. Every Cave Ticket spent there is a ticket you cannot use on Crystal Cave or Rune Cave runs. Never use tickets on free content.
Not checking the forge between runs. The sword drops from Dragon Tombs, but its real power comes from forge upgrades. A +0 Fiery Iron Sword is fine for entry-level content, but a +5 Fiery Iron Sword with a levelled Fire Rune is a difference you feel in every dungeon clear.