Bring as much help as you can. The idea is to surround the Dragon so that you and each member of your party can each take a swing, while the Dragon only gets one swing per turn. There is a secret entrance (it is a large shortcut) into the Dragon's lair. It is in the South East corner of the lair.
You can usually get three elves to follow you. Try to put one of the elves between you and the Dragon. Have the other two elves stand next to the dragon - another attempt to surround the Dragon.
Hopefully you are wearing (and have not damaged) the golden Armor of Zoth. The fire Sword of Abrial helps too. Along with a Wand and a Blaster.
Did you bring your "Summon Demon" spell? If you summon a demon, he will help you fight - just like the elves, but he is more powerful.
Did you bring your "Animate Bones" spell? Bring a bunch of bones into an area just outside of the Dragon's lair, and use the spell. Each pile of bones becomes a skeleton to help you fight the Dragon.
