Buy OSRS Extended super antifire(4)
The Extended super antifire(4) is a members-only potion in Old School RuneScape. It gives you full immunity to dragonfire for six minutes per dose. A full 4-dose potion lasts 24 minutes. That is a long time to fight dragons without taking fire damage.
This potion is great for long trips to dragon slayer tasks or metal dragons. Keep in mind that it does not stop Galvek's attacks. It also does not block the King Black Dragon's poison, shock, or freeze breath. Against Vorkath, it only blocks part of his attacks. You must have started Dragon Slayer I to use it.
Making this potion yourself is hard. You need level 98 Herblore and the recipe from Primula in the Myths' Guild, which needs Dragon Slayer II to enter. It takes a super antifire potion plus four lava scale shards to make a 4-dose potion.
The game warns you in your chatbox when the potion is about to run out. A second message appears about 15 seconds later when it has expired, so you have time to sip another dose and stay safe.