Enter Bihar Mines and use the carts to kill 10 enemies. This must be done in one trip through the mines. Leaving and reentering will reset this acheivement. Carts are moved by hitting them with a sword. You can kill multiple enemies with one cart but a poorly-timed hit will cause a cart to hit your party members, which will kill them as well. Note that you can only enter Bihar Mines after completing Vesplume Tower.