Edge Flat Sword with a 74% Soil Attr. Edge Katana refined 4 times with -5 DX
If the Flat sword (AP*0.74) is greater than Edge Katana AP, just use the Flat sword. If it is lower, depends on your usage. If you just ran around whacking things, stick with Flat Sword. If you use Earthquake Blade, keep the Flat Sword.
Other, put both of them in your quick slot, so you can change easily between the two by pressing F1 and F2 (example).