Though might confuse unprepared player with bizarre amount of incoming damage, dragon is pretty simple to deal with. You will need exactly 8 teleport scrolls and 5 invulnerability potions (that if you have 5lvl health sword, amount might vary). When approaching dragon activate potion. Watch closely when it expires, and the second it does - hit teleport spell. Then when cooldown on scrolls refreshes click it again. After second time potion cooldown will reach zero. And you'll be able to apply another. Rinze and repeat till dragon is defeated.