On any level in the mines, save your game and turn off the DS, then restart it and load your data. You will load onto the floor you saved on, but all monsters will be GONE. Please note, if you progress any further in the mine, all new monsters will sometimes be INVISIBLE, but are still there (the mines reset after you leave, so it isn't permanent). This is really useful when getting to deep floors and being able to get tons of items hassle free.
