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.