First, you need Lackche to have major Odo blood by pairing with Holyn, or she will not be able to wield it. To start the glitch, equip Shanon with the Balmunk, and get killed. Have Corple use the Valkyrie Staff, and press L or R on Shanon. Shanon's Balmunk should change to an Iron Lance. Now Save your game, and reset. Load your game back up, and go to the Shop in your castle. Balmunk should be for sale. Now have Lackche buy it. Normally she wouldn't be able to buy it, but if you did the above steps it should work.