Find a dragon, then cast "MON ZO GOR SAR" and press 'ESC' to pause the game. Hold down 'ALT' and type "LORD LIBRASULUS SMITHES THEE DOWN". Unpause the game and slay the dragon. He should leave a firestaff for you and you should be invincible.
How to kill any dragon:
Using the Vorpal Blade:
If you don't have both Vorpal Blades, you can increase the number of hits per time by passing the weapon back and forth between the two attacking players. Attack with one player and pass blade to another player and have this player attack. Now pass the blade back and attack. Continue until the monster is dead.