After killing a boss, take the sword and go back to the Twist Space. Don't save! Just turn off the game and reload. The game will put you on the room after boss with your new sword, but if you return to the previous room the boss will be there again. Each time you repeat this procedure you will gain one more sword. Repeat this as many times as desired, you can do this until he drops a very good sword, or just repeat 100 times to sell the swords later (each one can be sold for 1000 gold).