To easily get the Thief job, have at least the level 2 Black Mage. When you pick 
up the rare item from Merchant X (Thief's Memory) inside the Guardian of Flame 
dungeon (the glowing ring on floor 11 that will either take you to a duel or to 
a shop), walk away from Dungeon Hero X until he is close enough to use Sleep on. 
Then, run to the stairs. Try to run around the south side of the shop to make 
this easier. If Dungeon Hero X is gaining power, move two steps to the side of 
him. His beam is three spaces wide and goes very far. Keep Dungeon Hero X asleep 
all the time, if possible. This may take a few attempts, but it is much easier 
than any other way. If you get Dungeon Hero X asleep at the very start, you may 
be able to run directly to the stairs without going south around the shop, but 
this is much riskier. Since Dungeon Hero X can move much quicker than you, try 
to stay as far away from him as possible, but also making sure he is asleep.