Raising weapon mastery

[Norbac]

Fastest way?
In disgaea 5 you could max them in no time with juice and iirc the jumping minigame, there doesn't seem to be a shortcurt this time?

[Part-Time Prick]

12-5 has always counter spaces, raise your counterattacks on every slot on your characters, pass the bills, etc, and equip the evility cloth of truth and calm aura on a couple people.
They will proceed to beat the ♥♥♥♥ out of each other in a massive counterloop to gain weapon and armour experience.
Obviously the raise proficiency to S rank evilities help this along greatly,
That's about it though.

You can obviously use the existing enemies for the same thing but if you're trying to level a bunch of people anyway may as well have a bunch of people with 12 counterattacks going at it.

[Hanzo]

Get calm aura and forced counter. Then divide your party to team of 2 and beat each other. Level 1-2 is good where is not much interference.