Most of the Bokutos with -15 MW were reforged exceptional Bokutos with an Ash Runic Dovetail Saw. Oak will not work. Choose Powerful Reforging, and all four Artifice groups. For groups, select (1) Arcane/of Wizardry and (2) Exquisite/of Quality.
You will go through a bunch of tools so be prepared for a lot of garbage before you get a clean -15 MW bokuto.
If you want to do this on metal weapons (maybe a kryss) you will need to use a Copper Runic or higher with Powerful reforging and the same artifice options.
Oh, and then the last steps after you reforge the -15 MW is to POF, imbue, and then enhance (either with Frostwood for SC/FC-1 or Bloodwood for HPR2/HLL16). I personally would use a Forged Artifact tool to guarantee your enhance.
-OBSIDIAN-